In order to understand the effects of body weight-bearing on knee joint biomechanics, a video-fluoroscopic digitizing system was applied and in vivo human knee joint motions of 12 healthy adults under body weight-bearing and no weight-bearing conditions were recorded, digitized and analyzed. The changes of following parameters, including the knee flexion angle versus time, the length of the moment arm of the quadriceps muscle, the ratio between patellar tendon force and quadriceps muscle force, the ratio between patellofemoral joint reaction force and patellar tendon force in the sagittal plane during knee flexion-extension exercises under weight-bearing and no weight-bearing conditions were measured and compared. The results revealed that biomechanics of the knee joint motion under weight-bearing and no weight-bearing conditions were quite differnet. The knee under body weight-bearing condition extended rapidly and decelerated at near6-extension but the knee without body weight-bearing extended and flexed at almost equal speeds. The moment arm of the quadriceps muscle was shorter in knee exercise with body weight-bearing than that with no body weight-bearing but two groups showed similar patterns of change. The ratio between the tension of the patellar tendon and the quadriceps muscle force changed less obviously in the group performing knee exercise under body weight-bearing than the group performing knee exercise without body weight-bearing. The ratio between the patellofemoral joint reaction force and the quadriceps muscle force changed more obviously in the group with body weight-bearing than the group without body weight-bearing. Therefore,we concluded that body weight-bearing has strong influence on knee joint biomechanics. Besides, we should be more judicious in conducting in vitro studies of knee biomechanics because body weight-bearing is definitely a determining factor.