Can I extend a visa on arrival in Thailand for an additional 30 days?

@D**

as Robert pointed out, depending on your nationality AND the way you entered thailand you MIGHT be able to get a 30 day extension.

If you got stamped in for 15 days on a "visa on arrival" you CANNOT get a 30 day extension. <- A visa on arrival is something that people from 21 countries buy (although right now they're free) when they enter thailand. It's good for 15 days.

The countries that get 15 day visas on arrival are:

Andorra, Bhutan, Bulgaria, China, Cyprus, Ethiopia, Fiji, India, Kazakhstan, Latvia, Lithuania, Maldives, Malta, Mauritius, Paupa New Guinea, Romania, San Marino, Saudi Arabia, Taiwan, Ukraine & Uzbekistan

So if you're not from one of those countries you didn't get a visa on arrival when you entered the country.

How about stating the country your passport is from so people can actually give you accurate information?

But we are nice people here. If you story on your profile is correct, you come from Ramat Gan, a part of Tel Aviv that is in Israel. I guess you have an Israeli passport, with this passport you can get a 30 day Visa Exempt Entry and 30 days Extension of Stay at Immigration Office. %20on%20Arrival/VOA_%E0%B9%81%E0%B8%81%E0%B9%89%E0%B9%84%E0%B8%82%2015-03-61.pdf

Simple answer.Yes, but as we do not know the country of origin of passport it could be: No you can not. .Basic information is needed to give advice.

If you buy visa on arrival for 15 days, you cannot. If you get 30days visa exempt stamp for free, depends on your passport country, some can get 30 days extension for 1900 baht at your local immigration office, some can get 7 days, some cannot.

is not sure some national can not

Yes at your nearest immigration office for 1900 baht.