Abstract: The influence of cryogenic mediums (liquid nitrogen and liquid oxygen, LN₂ and LO₂) on the mechanical properties of carbon fiber reinforced epoxy resin (CFs/EP) was studied. T700 carbon fibers (CFs) and CFs/EP were immersed in liquid nitrogen (LN₂) and liquid oxygen (LO₂) for 120 h. The surface elements and morphology of T700 CFs were investigated by XPS and SEM, respectively. The mechanical properties of T700 CFs and CFs/EP were also researched after immersion in LN₂ and LO₂. The results show that there are obvious damages on the surface of T700 CFs after immersion in LN₂, but no damages are found on the surface of T700 CFs after immersion in LO₂. The different influence of LN₂ and LO₂ on the surface elements of T700 CFs and single fiber tensile strength could be negligible. After immersion in LN₂ and LO₂, the tensile strength of CFs/EP decrease 15.26% and 14.38%, respectively, and the flexural strength of CFs/EP increase 56.18% and 57.81%, respectively. Consequently, the influence of LN₂ and LO₂ on the mechanical properties of CFs/EP are nearly no difference.