Postal codes in Thailand are five digit numbers. The first two digits of the postal code denote the province or special administrative area (e.g., 43120 Phon Phisai, Nong Khai).
Listed below are the first 2 digits of codes assigned to each province or special administrative area.
Special Governed Districts (Khet Phiset)
References
Postal codes in Thailand Wikipedia(Text) CC BY-SA