Abstract: After the total knee arthroplasty (TKA), a fracture in the area from the lower lateral to the upper medial of the distal femur can sometimes be found. The causes of the fracture are investigated in this paper, through analyzing the plastic strain and stress distributions in the distal femur in the conditions of standing, knee-bending and walking. The plastic strain in the femoral condyle is observed in the intercondylar fossa and near the edge line of the prosthesis under the three conditions. During the walking, the plastic strain is mainly observed on the lateral side of the femoral condyle, extending to the upper medial part, whose orientation is consistent with the fracture line. This is the important cause of the distal femur fracture in the area from the lower lateral part to the upper medial part. Moreover, the difference between the lateral stress and the medial stress in the distal femoral shaft is especially important, and the lateral high stress concentration area is near the femoral condyle, which is another important reason that leads to this kind of fracture.