Elastic modulus of composite materials during fatigue tests was evaluated from short term creep tests and a modified visco-elastic model. In the elastic region of tensile tests, the fiber surface treatment did not affect stress-strain relations. The surface treatments, however, changed fatigue life, i. e., depending of surface treatment, difference with tens of times of fatigue life at the same stress amplitude was observed. In fatigue tests, the change of elastic modulus, evaluated from the modified visco-elastic model, had good correlation to residual strain. Except the last stage of the fatigue process, cracks were not observed in resin. Therefore, fatigue damage is considered to consist of initiation and propagation of interfacial debonding at the end of fibers, and increasing of residual strain which came from fiber pulling-out. From interfacial debonding length, which was evaluated from the reduction of elastic modulus, the effective fiber length was considered to be shortened by more than 50%.