Anterior Cruciate Ligament Injury often occurs as a result of knee valgus collapse during landing or pivoting in sports activity. Previous studies reported that the risk of ACL injuries was reduced by jump and balance training, and those training can be effective as the prevention program for the ACL injuries. But those studies often focused on only adult athletes, and there are few studies focused on junior athletes. The purpose of this study is to investigate the pattern of landing movement in junior athletes, and to verify the effect of the prevention program. One-hundred and ten junior basketball players (boys; 61, girls;49, age ranging 12 to 15) were subjected in this study. We measured the knee flexion, valgus angles and jump height during continuous vertical jump. After measuring, they executed a prevention program for 12 weeks. The motion pattern of the knee during jump test were compaired between before and after prevention program. Female athletes showed greater knee valgus angle at initial landing phase and grater maximum knee valgus angle than those of males. As a result of the prevention program, maximum knee valgus angle was significantly decreased in female athletes. The greater angle of knee valgus in female may increase the risk of ACL injuries. Present study suggests that the prevention program is useful for reducing the risk of ACL injury. Increment of jump height in male players after this program might be considered that this training program has also an effect of performance improvement.