The VISA ON ARRIVAL allows passport holders of 19* countries to enter Thailand under this rule provided that they meet these requirements:
The visit is strictly for tourism purposes.
They must have a confirmed return ticket to show that they are flying out of Thailand within 30 or 15 days of entry, as appropriate. Open tickets do not qualify. Travelling overland out of Thailand by train, bus, etc to Cambodia, Laos, Malaysia (including en route to Singapore), Myanmar, etc is not accepted as proof of exiting Thailand.
You may be asked to show your flight ticket on entering Thailand. If you do not possess a flight ticket to show you will be exiting Thailand within 30 or 15 days of entry you will be most likely to be refused entry.
It will also be necessary to prove that you have funds of at least 10,000 THB per person and 20,000 THB per family during your stay in Thailand. A fee of 1,000 THB is payable upon entry and is subject to change without notice.
However, persons who are not eligible to enter Thailand under the VISA ON ARRIVAL and VISA EXEMPTION RULE are advised to obtain visas with the Royal Thai Embassy prior to their visit.