This paper deals with the effect of water environment on the fatigue behavior of fiberglass reinforced plastics. The time-dependent phenomenon from the penetration of water to the absorption into the bulk is considered to be a diffusion process of water molecules which is accelerated by the internal defects formed under the repeated stresses. The debonding at the interface between fiber and resin matrix was observed with the variation of the diffusion coefficient due to the progressive fatigue damage. This variation depends on the angle, θ, between the warp and the load direction. When θ = 0 deg, the weakened interface due to the water absorption causes the fibers to carry more of the load and plays an indirect role in accelerating the fatigue damage. On the other hand, water directly acts to extend the shear failure of the interface when θ = 45 deg, so that the fatigue life especially decreases. These fatigue behaviors were proved through observations by means of a scanning electron microscope.