To investigate the effects of string tension and racket size on ball rebound velocity, potential energy of the strained string during a dynamic impact was analysed. The hitting tests were also conducted in order to clarify the effects. In this study, regular and large-sized rackets were used. Each type of racket was strung with synthetic string at tensions of 25〜65 1b (111〜289N). The results obtained are as follows: 1) The rebound velocity depends on the hitting location of the racket face. And the peak point of velocity coincides approximately with the "sweet point" empirically estimated. 2) The rebound velocity increases as the string tension decreases in ordinary condition. 3) The large-sized racket has wider sweet area and larger rebound velocity than the regular-sized. 4) The potential energy of string during impact has considerable effects on the ball rebound velocity.