Under Thailand Law, Businesses in Thailand for Thai people may be registered as sole proprietorships, limited and unlimited partnerships, limited companies, public companies, branch offices, or representative and regional offices.
However, for foreigners, The Foreign Business Act 1999, formerly known as The Alien Business Law, restricts business activity of non-Thai "aliens" by classifying businesses into three categories: Businesses in list one are generally closed to foreigners. Businesses in lists two and three generally require an Alien Business License for a foreigner to own and operate them.
Since 1966, The Kingdom of Thailand and the United States of America have entered U.S.-Thai Treaty of Amity and Economic Relations of 1966 ("Amity Treaty") which exempt American Citizens either by birth or naturalization from many restrictions imposed by the Alien Business Law of 1972.
The Treaty allows American companies to maintain a majority shareholding or own its company 100%, branch office or representative office located in Thailand. They may engage in any businesses on the same basis as Thai companies in the USA. Amity Treaty Companies are not allowed to own land or engage in certain enterprises such as domestic trade in agricultural products, land and natural resources exploitation and the liberal professions. However, the disadvantages to Amity Treaty Companies is Amity Treaty companies take longer and are more expensive to register than both Thai companies and non-Amity Treaty foreign majority companies. Amity Treaties also operate under a Foreign Business license and therefore generally under greater scrutiny than Thai majority companies.
Businesses registering through the Treaty of Amity do not have to receive approval from the BOI but it may be advisable that they do so in order to attain some of the other benefits available from the BOI. Nonetheless, certain businesses, such as those in List One and the liberal professions, are still prohibited even to American companies.
In return, Thais are extended reciprocal rights to invest in the U.S., and Thai businesspersons are eligible to receive U.S. visas as "treaty traders" and "treaty investors". Under the Treaty, Thailand is permitted to apply the following restrictions to American and other foreign investment: owning land; engaging in the business of inland communications; inland transportation; fiduciary functions; banking involving depository functions; engaging in domestic trade in indigenous agricultural products; and exploiting land or other natural resources
Application for certification of a Treaty company is conducted with or after establishing a Thai company. It normally takes less than one week for certification and may take 1-3 months to complete.