The purpose of this study was to investigate the effect of racket size, string material and string tension on the coeffient of restitution (COR) for tennis rackets. In order to get accurate results, the racket was fixed with the clamp and the tennis ball was controlled to impact the racket vertically. The whole experimental procedure was filmed by NAC high speed camera at the rate of 1000 frame per second. The video tape was digitized using Perk Performance System, and the date were then analyzed with three-way ANOVA. The result of this study were summarized as followed: 1. For tennis racket, the interaction effects among the racket size, string material land string tension are not significant, but each individual factor (racket size, string material, or string tension) has significantly effect on the COR value. 2. The COR value of 40-1b string tension racket is significantly higher than those of other string tension for tennis. 3.The COR value of over-size racket is significantly highetr than of small size racket for tennis. 4.The COR value of cotton string racket is significantly higher than of nylon string racket for tennis.