Passports & Visas

           All visitors to Thailand must carry a valid passport with proof of onward passage (either a return or through ticket). Visa applications are not required if you are staying less than 30 days and are a national of 1 of 41 designated countries including Australia, Canada, Ireland, New Zealand, the United Kingdom, and the United States (New Zealanders may stay up to 3 months).

           The Immigration Division of the Royal Thai Police Department is found at 507 Soi Suan Plu (off Sathorn Road, and a short taxi ride from Chong Nongsi BTS; tel. 02287-3101). Many foreigners have made use of the limitless 1-month free entry visas that many travelers used to be granted on arrival. However, since November 1, 2006, this free visa privilege only extends to an accumulated total stay of 90 days, in a 6-month period.

           Visitors planning to stay for longer than a month can arrange for 60-day tourist visas at embassies overseas for a cost of 1050B ($30/£16); this is renewable in Thailand for an additional 30 days for another 1,900B (US$54/£29). If you overstay your visa, you will be charged between 500B and 1,000B (US$14-US$29/£7-£15) per day, which is payable when exiting the kingdom. Longer overstays are punishable by anything upwards of a 20,000B (US$571/£308) fine or a stay in jail.

Source:

http://www.frommers.com/destinations/thailand/0245020059.html