The purposes of this study were to investigate the bat-head velocity produced by the torso and upper extremity rotation in baseball batting and to identify the kinematic factors influencing the between-subject variance in bat-head velocity. Seventeen male collegiate baseball players were asked to perform tee-batting with maximal effort. A motion capture system operating at 500 Hz was used to record batting motion. A simplified mechanical model composed as five rigid segments (pelvis, thorax, shoulder girdle, upper extremity and bat) interconnected by smooth spherical joints was developed to calculate the bat-head velocity produced by each joint using the method described by Sprigings et al. (1994). The order of the mean values of the bat-head velocity at the ball impact were the wrist joint (27.3 m/s), the lower torso joint (4.6 m/s), the pelvis rotation (4.6 m/s), the upper torso joint (1.7 m/s), and the pelvis translation (0.2 m/s). However, only the shoulder joint showed negative value (–3.2 m/s). In order to maximize bat-head velocity, it is important to suppress an increase in bat-head velocity during start-up period of swing phase by preventing rotation of shoulder joint, and soon after that to sharply increase bat-head velocity during early period of swing phase by rotating the thorax in time for the pelvis rotation.