The thermal stability of oxygen-containing functional groups on activated carbon surfaces in a thermal oxidative environment was studied. The raw activated carbon (ACO) was first treated with nitric acid, and the resulting nitric acid-treated activated carbon (ACn) was further oxidized under 2.5% O-2 (in N-2) atmosphere at different temperatures. The types and the amount of oxygen-containing functional groups were analyzed by thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy (FTIR), Boehm titration, and X-ray photoelectron spectroscopy (XPS). Both oxygen-and nitrogen-containing functional groups were introduced onto the ACn surface. Under thermal oxidative conditions, hydroxyl was oxidized to the corresponding carboxyl group in the temperature range of 378-473 K, and epoxy groups and lactones were generated between 573 to 773 K via oxidation reactions between graphitized carbon and oxygen. In contrast, carboxyl decomposition occurred at around 573 K. Lactones, ketones, and quinones exhibited better thermal stability, undergoing decomposition between 773 to 973 K. Ether and epoxy groups exhibited the best thermal stability, decomposing only at temperatures above 973 K.