Regarding traveling to work in Thailand legally by foreigners, there are four categories of foreign workers who are eligible to apply for work permits in Thailand, as follows: 

1. Temporary: General foreign workers (Section 7) who have a residence in the Kingdom and foreign workers who have been permitted to enter the Kingdom temporarily (non-immigrant visa);
   
   
2. Investment Promotion: Foreign workers who are permitted to work in the Kingdom under the law on investment promotion or according to other laws that have provisions about foreign workers in the same manner, such as the Petroleum Act, the Industrial Estate Act (under Section 10);
   
   
3. Foreign workers under Section 12, in four cases:
   3.1 Foreign workers who have been ordered to be deported under the law on deportation, but who have been granted leniency to engage in employment at s certain place instead of deportation, or who are pending deportation;
   3.2 Foreign workers who come to live in the Kingdom without permission according to the law on immigration and pending repatriation from the Kingdom, such as Vietnamese, Laotian, Nepali, Myanmar, displaced, or foreign workers, according to the Cabinet resolution on 29 August 2000; 
   3.3 Foreign workers who were born in the Kingdom but did not acquire Thai nationality in accordance with to the Announcement of the Revolutionary Council No. 337 dated 13 December 1996 or other laws, such as persons born after 13 December 1972, which was the date on which the Announcement of the Revolutionary Council became effective;
   3.4 Foreign workers whose nationality was revoked, according to the Announcement of the Revolutionary Council No. 337, dated 13 December 1972. 
   
   
4. Permanent: Foreign workers permitted to work according to the Announcement of the Revolutionary Council No. 322, item 10 (1). There is material concerning the permit issued to a foreign worker who has been residing in the Kingdom under the law on immigration and already working before 13 December 1972, which can be used for the entire life of that foreign worker, unless the foreign worker changes to a new career.

Source :Foreign Workers Administration Ofhce
Tel : +66 63 663 6663