Abstract: To investigate the overall response of MCL rupture on the biomechanics of a tibiofemoral joint under a valgus moment, the MRI images of the normal human knee at flexion angles of 0°,25°,60° and 80° were developed through SONATA MAESTRO 1.5T and the intact and MCL-deficient models were built, based on these images. Then different valgus moments were applied to these models for the finite element simulation to obtain the role of ligaments as well as load transmission and state of stress in various components. The results show that: 1) MCL rupture causes larger tensile forces in the cruciate ligaments in valgus rotations, especially in ACL. The resistances assumed by cruciates gradually increase following flexing angle increases. Then the cruciates, especially the ACL take a big risk to hurt at high knee flexion angles when MCL ruptures. 2) The MCL-deficient joints fail to maintain enough joint stability in distal-proximal translations, and the resultant changes in joint laxity between femur and tibia, especially in medial much larger during valgus stress loading. The lateral articular cartilage and lateral meniscus experience a much larger compressive force and the percentage of found in femur cartilage at 25°, in tibial cartilage and meniscus at 25°and 80° in the MCL-deficient models, suggesting that the tissues may be most vulnerable to injury at these flexion angles under valgus loading conditions. the load transferred through the meniscus gets smaller. 3) The highest measured effective stress changes were found in femur cartilage at 25°, in tibial cartilage and meniscus at 25°and 80° in the MCL-deficient models, suggesting that the tissues may be most vulnerable to injury at these flexion angles under valgus loading conditions.