The fatigue property of the composite resin has recently been a subject of research. In this study, the fatigue strength and the tensile strength of composite resins stored in water were investigated. Two types of light cure composite resins were used: a Hybrid type and am MFR type. Dumbbell-shaped specimens were prepared. After storage at 37 +/- 0.5 degrees C for 24 hours, 1 month, 3 months, 6 months and 12 months, the fatigue test and the direct tensile test were performed. The staircase method was employed to analyze the fatigue strength under a 10(5) cyclic load. The fatigue strength of the Hybrid type at 24h ours was 54.7 MPa; that of the MFR type was 28.1 MPa. Both fatigue strengths decreased during water storage. Otherwise, the tensile strength of the Hybrid type was 77.0 MPa; that of the MFR type was 53.5 MPa. After 12 months of storage, the tensile strength of the Hybrid type remained unchanged during water storage; however, that of the MFR type decreased.