Accelerated aging tests on the durability performances of carbon fiber reinforced plastics (CFRP) bars and glass fiber reinforced plastics (GFRP) bars were conducted under different conditions, including acid, alkaline, and sulfate environments with UV light irradiation at various temperatures and humidity. The results showed that the CFRP bars exhibited excellent durability performances, but the GFRP bars were not as good as the CFRP ones in terms of durability performance. The reduction factors of durability performance derived from this work may be applied to the design of civil engineering structures that employ FRP bars.