Carbon fibers-reinforced polyimide composites (CF-PI) were fabricated by means of a hot press molding technique. To contrast the effects of ultraviolet and atomic oxygen irradiation under high vacuum on the tribological properties of CF-PI composites, the friction and wear properties of the composites sliding against GCr15 steel ball before and after irradiation were conducted in high vacuum on a ball-on-disk test rig. The experimental results revealed that CF-PI composites exhibited higher modulus and lower coefficient of friction and worn rate value than pure polyimide under high vacuum. However, the coefficient of friction of composites increased and the worn rate value decreased after ultraviolet or atomic oxygen irradiation, which slightly affected the tribological properties of CF-PI composites. The chemical composition of the composites changed after irradiation was inspected by X-ray photoelectron spectroscopy. Microstructure of the worn surfaces of the tested composites was investigated by scanning electron microscopy to reveal the wear mechanism.