The magnitude of mechanical stress at the shoulder and elbow appears to be directly correlated with the degree of maximum shoulder external rotation (MER) during throwing. Therefore, it is very important to prevent excessive MER to minimize the risk of throwing injuries. The purpose of this study was to investigate the relationships between MER during throwing and the kinematic parameters of throwing mechanics, shoulder muscle strength, and shoulder range of motion in high school baseball players. The subjects were 40 male high school baseball players with no elbow or shoulder joint problems. Three-dimensional analysis was performed to calculate the MER angle. Then, the shoulder and elbow angles at initial foot contact (IFC) were computed. ROM and muscle strength of shoulder joint were also measured in each subject. Multiple linear regression analysis was used to relate the MER angle to these factors. Significant correlations were observed between the MER angle and the external rotation (ER) angle (r=−0.51, p<0.001) at IFC, and the ER range of motion (r=0.84, p<0.01). The MER angle significantly correlated with shoulder internal rotation (IR) at IFC. This finding suggests that stress on the shoulder and elbow could be increased by the degree of shoulder IR angle at the moment of IFC. Further, excessive ER range of motion may also be a risk factor.