To observe the change in the status of stresses according to size of the meniscus, a 3-dimensional photoelasticity model of the knee joint was made of epoxy. Three kinds of meniscus models were made of rubber. Through the axial application of a vertical compressive load of 8kg equivalent to the joint reaction of 3,000N in the human knee joint, the peculiar patterns of the isochromatic fringes were observed and stresses around the knee joint were analyzed according to the size of the defect in the medial meniscus. Even distribution of the photoelasticity fringe patterns were observed in the normal knee joint model with both menisci intact. Stress concentration was increased at the margin of the medial side of the knee joint model with the medial menicus removed partially in about 30-50% of its central portion. In addition, the magnitude of stresses were also noted as increased in the lateral part of this type model. In the knee joint model with nearly all of the medial meniscus removed, strssess were concentrated markedly in the central portion of the knee joint just outside of both tibial spines. Stress was increased in its magnitude according to the size of the defect of the meniscus, and was focalized after menisectomy. In the model for partial menisectomy, the maximum stress concentration point of the removed side migrated to the margin of the same side of the joint. But in the model of total menisectomy, maximum stress concentration points of both side were more centralized. Not only an increment of stresses in magnitude but also centralization of the maximum stress concentration point in its location could contribute to the degenerative process of the knee joint after menisectomy. In addition, articular cartilage can be easily damaged with a relatively small amount of external force.