The restitution characteristic of fiber string in axial direction was analyzed with considering the visco-elasticity of yarns. An impact test was developed that one side of string was fixed in load cell, the other side was fixed in hammer. The hammer moves along the slide rail. When the hammer was struck, the impact speed of hammer was measured with the laser sensor, and the restoration ratio was obtained. In experimental range for all samples, the restoration ratio became small with the impact speed. A dynamic restitution model considering tensile non-linear visco-elasticity of materials was suggested. The good agreement was obtained between the experimental and the theoretical values. It was also shown that the repulsion of tennis strings could be evaluated with the parameter of the proposed model.
