1. Generally, a foreign citizen who wishes to enter the Kingdom of Thailand is required to obtain a visa from a Royal Thai Embassy or a Royal Thai Consulate-General. However, nationals of certain countries do not require a visa if they meet visa exemption requirements as follows:
(1) they are nationals of countries which are exempted from visa requirements when entering Thailand for tourism purposes. Such nationals will be permitted to stay in the Kingdom for a period of not exceeding 30 days. For more information, please see Tourist Visa Exemption;
(2) they are nationals of countries which hold bilateral agreements with Thailand on the exemption of visa requirements. For more information, please see List of Countries which have Concluded Agreements with Thailand on the Exemption of Visa Requirements .
2. Nationals of certain countries may apply for visa upon arrival in Thailand. Travellers with this type of visa are permitted to enter and stay in Thailand for a period of not exceeding 15 days. For more information, please see Visa on Arrival.
3. Travellers travellling from/through countries which have been declared Yellow Fever Infected Areas must acquire an International Health Certificate verifying the receiving of a Yellow Fever vaccination. For more information, please see List of Countries which are Declared Yellow Fever Infected Areas.
4. Nationals of certain countries are required to apply for a visa only at the Royal Thai Embassy or the Royal Thai Consulate-General in the applicant�s country of residence, or at the Royal Thai Embassy which has jurisdiction over his or her country of residence. Travellers are advised to enquire about authorised office for visa issuance at any Royal Thai Embassy or Royal Thai Consulate-General before departure. Contact details and locations of Royal Thai Embassies and Royal Thai Consulates-General are available at www.thaiembassy.org
5. To apply for a visa, a foreigner must possess a valid passport or travel document that is recognised by the Royal Thai Government and comply with the conditions set forth in the Immigration Act of Thailand B.E.2522 (1979) and its relevant regulations. In addition, the visa applicant must be outside of Thailand at the time of application. The applicant will be issued with a type of visa in accordance to his or her purpose of visit. For more information on types of visas and general requirements for each type of visa, please see Types of Visa and Issuance of Visa.
6. In general, applicants are required to apply for a visa in person. However, Royal Thai Embassies and Royal Thai Consulates-General in some countries and in some cases may also accept applications sent through representatives, authorised travel agencies or by post. Please enquire at the Royal Thai Embassy or Royal Thai Consulate-General where you intend to submit your application of acceptable ways of application.
7. Please note that the period of visa validity is different from the period of stay. Visa validity is the period during which a visa can be used to enter Thailand. In general, the validity of a visa is 3 months, but in some cases, visas may be issued to be valid for 6 months, 1 year or 3 years. The validity of a visa is granted with discretion by the Royal Thai Embassy or Royal Thai Consulate-General and is displayed on the visa sticker.
8. On the other hand, the period of stay is granted by an immigration officer upon arrival at the port of entry and in accordance with the type of visa. For example, the period of stay for a transit visa is not exceeding 30 days, for a tourist visa is not exceeding 60 days and for a non-immigrant visa is not exceeding 90 days from the arrival date. The period of stay granted by the immigration officer is displayed on the arrival stamp. Travellers who wish to stay longer than such period may apply for extension of stay at offices of the Immigration Division 1 in Bangkok, located at Government Center B, Chaengwattana Soi 7, Laksi, Bangkok 10210, Tel 0-2141-9889 or at an Immigration office located in the provinces. For information on application for extension of stay, see the Immigration Division 1 website at www.bangkok.immigration.go.th
9. Foreigners entering Thailand are not permitted to work, regardless of their types of visa, unless they are granted a work permit. Those who intend to work in Thailand must hold the correct type of visa to be eligible to apply for a work permit. Information on Work Permit applications could be obtained from the website of the Office of Foreign Workers Administration, Department of Employment, Ministry of Labour
10. Royal Thai Embassies and Royal Thai Consulates-General have the authority to issue visas to foreigners for travel to Thailand. The authority to permit entry and stay in Thailand, however, is with the immigration officers. In some cases, the immigration officer may not permit foreigner holding a valid visa entry into Thailand should the immigration officer find reason to believe that he or she falls into the category of aliens prohibited from entering Thailand under the Immigration Act B.E. 2522 (1979).
11. According to the Immigration Act of Thailand B.E. 2522 (1979), foreigners who fall into any of the following categories are prohibited to enter Thailand:
(1) Having no genuine valid passport or document used in lieu of passport; or having a genuine valid passport or document used in lieu of passport without valid visa issuance by the Royal Thai Embassies, the Royal Thai Consulates-General or the Ministry of Foreign Affairs, with exception of those who meet visa exemption requirements. The terms and conditions of visa issuance and visa exemption are prescribed by the Ministerial Regulations.
(2) Having no appropriate means of living following entry into the Kingdom.
(3) Having entered the Kingdom to be employed as an unskilled or untrained labourer, or to work in violation of the Alien Work Permit Law.
(4) Being mentally unstable or having any of the diseases stated in the Ministerial Regulations.
(5) Having not yet been vaccinated against smallpox; or inoculated, or undergone any other medical treatment for protection against disease; and having refused to have such vaccinations administered by the Immigration Doctor.
(6) Having been imprisoned by judgment of the Thai Court; or by lawful injunction or judgment of the Court of a foreign country, except for when the penalty is for a petty offence, or negligence, or is provided for as an exception by the Ministerial Regulations.
(7) Having behaviour which could cause possible danger to the public; or having the likelihood of being a nuisance or constituting any violence to the peace, safety and security of the public or to the security of the nation; or being under warrant of arrest by competent officials of foreign governments.
(8) Reason to believe that entry into Kingdom is for the purpose of being involved in prostitution, the trafficking of women or children, drug smuggling, or other types of smuggling which are against public morality.
(9) Having no money or bond as prescribed by the Minister under Section 14 of the Immigration Act B.E. 2522 (1979).
(10) Being a person prohibited by the Minister under Section 16 of the Immigration Act B.E. 2522 (1979).
(11) Being deported by either the Government of Thailand or that of other foreign countries; or having been revoked the right of stay in the Kingdom or in foreign countries; or having been expelled from the Kingdom by competent officials at the expense of the Government of Thailand unless exemption is provided by the Minister on an individual basis.
source/ more info please go to Ministry of Foreign Affairs