Without a re-entry permit your extended visa will be cancelled any time you leave the country. Not only the extension will be invalid and void but also the visa itself will be invalid and void.

Requirements

Name	Info
Application Form TM8 (application must be made in person)
Copy of applicant’s passport's photo page	Passport Photo Page
Copy of applicant’s passport's current Thai visa page	Thai Visa
Copy of applicant’s passport's current entry stamp	Passport Entry Stamp
Photo of applicant, 4 x 6 cm, colored, no glasses. Not older than 6 months.	Photo
฿1,000 in cash for single entry and ฿3,800 in cash for multiple entry permit

Note

The re-entry permit does not prolong your visa or permission to stay. It is just a permission to return to Thailand using the remaining validity of your visa or visa extension.

Remember when arriving back into Thailand, you must write the re-entry permit number on your Arrival Card as "Visa no.":

Arrival and Departure Card (TM6)