The purpose of this study was to investigate the effects of racket stiffness and string tension on the coefficient of restitution (COR) in tennis rackets. In the experimental test, the tennis racket was fixed in a C-clamp and the tennis ball was controlled by the ball shooting machine to hit the racket with a vertical impact at certain speed (approximated as 20 m/sec). NAC high speed camera (1000 frame/sec) was used to record the impact phenomenon. The video tape was digitized using Perk Performance System to get COR, and the data were then analyzed with two-way ANOVA. The result of this study are as follows:1. The higher stiffness of racket will acquire higher COR value.2. The COR value of 50lb string tension racket is significantly higher than those of other string tension rackets.