The purpose of this study was to investigate the effects of passive ankle and knee joint motions on the length of fascicle and tendon of the gastrocnemius muscle. The ankle joint was passively moved from maximal dorsiflexed position to maximal plantar flexed position, while the knee joint was fixed at 0° (fully extended position). Also, the knee joint was passively extended from 90° to 0°, while the ankle joint was fixed at 0° (neutral position). In each test condition, lengths of fascicle and external tendon of the medial gastrocnemius were measured by ultrasonography. The length of external tendon was significantly changed by the ankle joint motion, but not by the knee joint motion. This demonstrates that the length of the external tendon distal to the muscle belly is influenced only by the angle changes of the joint distal to the gastrocnemius muscle. The change in fascicle length during the knee joint motion was significantly larger than during the ankle joint motion in spite of a similar length change of total muscle-tendon complex between the two motions. This demonstrates that fascicles located proximally to the external tendon are more influenced by the angle changes of the joint proximal to the gastrocnemius muscle. These results reveal that angle changes of each joint located in the proximal and distal parts of a biarticular muscle have different effects on the length of fascicle and external tendon.