The efforts of this study were concerned with the degradation of polymer composite materials used in space environment. The investigations were focused on the low temperature and radiation damage effects. The material selected for study was carbon fiber reinforced epoxy composite. Experimental results showed that both low temperature (77K) and Co-60 radiation caused the decrease of mechanical strength of this polymer composite. For the highly radiation-degraded specimens, post-annealing treatment caused the formation of bubbles in epoxy matrix and the further weakening of the material. For the specimen with minor or without radiation, post-annealing treatment showed no influence for mechanical properties.