In order to evaluate weatherability of GFRP containing an unsaturated polyester composite, GFRP plates were exposed outdoors in Miyako-jima for 5 years. The GFRP plates were collected every year and thickness reduction, flexural strength and fatigue strength were measured. The thickness of GFRP after weathering test decreased linearly about 10 μm/year. The flexural strength of weather exposed GFRP had little changes compared with the unexposed GFRP. The fatigue strength after 2 years weathering decreased by 40% compared with the unexposed GFRP. However the changes of fatigue strength were small from 3 years on. In order to consider these tendency of fatigue strength, the effect of the surface roughness was evaluated. The surface roughness of weather exposed GFRP increased largely in 1 or 2 years and had little changes from 3 years on. It is confirmed that the tendencies of fatigue strength and surface roughness caused by weathering were similar. So it was considered that the fatigue strength reduction of weather exposed GFRP was caused by the changes of surface roughness due to weathering.