I have passports from myanmar (burma) and USA. if I apply for Thai visa ,can I use either one of the two ?

Just think if for some reason you used your Myanmar passport and got into trouble and was deported.

To be safe use your US passport, a Thai visa is not needed for social or tour purposes for US passport holders, of period not exceeding 30 days.

Though Burma is a member-state of the ASEAN. The latter has not yet concluded a Visa-Exempt Treaty with the Kingdom of Thailand. Thus, a Myanmese Passport holder would be required to apply for visa before entering Thailand.

Use only one passport, whenever you make entry to any country. When you use a US passport, you will use it in your entry and exit. AND I assume you are a dual citizen of the countries...yes, you can use either of the two passport, assuming further that you are recognized to be a dual citizen of Burma and the US at the same time.

But if your Burma passport was your former passport, and you are no longer a citizen of Burma due to your acquisition of US citizen, then you cannot use Burma passport. You must use your US Passport.

Myanmar may REQUIRE you to travel with your Burmese passport. Even if they do not, you MAY travel with it, and will likely encounter less scrutiny when you come to Thailand.

I am working on two assumptions:
1. You are ethnic Burmese
2. You are entering from Myanmar

If you are caucasion, the by ALL means use the American passport. Also, if you are traveling from a country outside of Myanmar and coming to Thailand, I would recommend the US Passport.

There are a high number of US citizens who hold dual nationalities.

It is VERY important note that use of a foriegn passport does NOT endanger U.S. citizenship. In other words, IF you were to run into problems during your trip to Thailand you would ABSOLUTELY NOT be deported back to Burma regardless of the circumstances, even if you did use a Burmese passport to gain access to Thailand.