This year i visited Thailand 2 times, both time i entered the kingdom with Visa on Arrival. I want to know if i want to visit again this year is there any restriction on visa on arrival. Will there any problem in obtaining the third time visa on arrival.  How the visa on arrival is extended to 30 days after entering Thailand.

• There is no limit and you will have no problem with a third Visa On Arrival.
• Visa on Arrival cannot be extended.

There are no restrictions but they cannot be extended.

Thanks very much, i asked this question through email to my nearest thai embassy, but they keep on sending me tourist visa application form.  Later i understood they didnt understand english.

The answers above are correct, but make sure you really do mean "visa on arrival" and not "visa exempt entry". Which you can get depends on your nationality, and the rules are totally different. You know you were using the correct terminology if you needed to pay for the visa on arrival. If you actually did mean "visa exempt entry", clarify that and we can tell you the rules.