Abstract: The short cut carbon fibers (CF) were treated by air oxidation and nitric acid oxidation，and the mechanical properties of CF reinforced thermoplastic polyimide(TPI) resin matrix composites (CF/TPI) were studied.The results of Boehm titration method show that the acidic functional groups on the treated surface of CF increase as the treating time extends，and the nitric acid oxidation is better than the air oxidation. Compared with the untreated composite，the tensile strength and flexural strength are improved by 10% and 14%，when the treatment time of nitric acid is 20 min. X-ray photoelectron spectroscopy (XPS) indicates that the acidic oxygen functional groups on CF increase by 35.89 %. The atomic force microscopy (AFM ) images show that the nitric acid is capable of increasing the roughness of CF. The scanning electron microscopy(SEM) photograph of the fracture surface reveals that there is interface bonding between treated CF and TPI .