FRP, one of the most general composite materials, is being used commercially more and more as a structural material. However, there have been only a few reports dealing with the degradation of inner structure of FRP due to fatigue. The present paper deals with this problem of degradation in FRP during the fatigue process. The FRP used in this study were GRP, CRP, and hybrid composites (which are composed of the former two). Firstly, the strength of GRP and its constituents was measured and the results were compared. Secondly, the residual strength of GRP and the change in strength of glass fiber strands in the matrix during fatigue, as well as the fatigue life of GRP, were investigated. The fatigue process of GRP was related to the degradation of the constituents in the matrix from the results mentioned above. After carrying out the similar experiments and discussion on CRP, the fatigue process of hybrid composites was discussed by comparing those of other two materials.