Recently, graphite/epoxy composite materials are quite common in military aerospace structures. In service or stock, composite structures may be exposed to natural environments for long periods of time. Therefore, environmental aging factors such as temperature, moisture, and UV radiation can reduce mechanical and physical properties of the composite material. In this study, accelerated aging test using Weather-Ometer was conducted to predict the long-term performance of a T300/AD6005 graphite/epoxy composite material system. To investigate the degradation of mechanical properties of the composite material and resin(AD6005 epoxy), tensile, compressive, shear, and flexural tests were performed at intervals of accelerated aging time of 500, 1000, 1500, and 2000 hours. It was found that the strength degradation of the composite and resin was much more severe than the stiffness degradation with the aging time. Matrix cracking and matrix loss were observed at the surface of aged specimens by using SEM. For correlation of accelerated aging to natural aging, acceleration factors were determined for several mechanical properties.