The purpose of this study is to make an emphasis the necessity of the dynamization after interlocking intramedullary nailing. We reviewed 75 cases of the fractures of the shaft of femur or tibia fixed with static interlocking intramedullary nailing. We excluded the cases in whom the locking screws were removed for the purpose of the relief of pain from the irriataion by those screws. The parameter we reviewed were the type of fracture, postoperative gap between the major fragments. Early callus formation, the union time, postoperative dynamization and controlled weight bearing. In those cases in which the contact surface between the major fragments was more than 50% and there noticed no early callus within 6 weeks after operation, the union time was shortened after dynamization, when the contact surface was less than 50% and there noticed no early callus, the union time was shortened after bone graft and dynamization which resulted in some shortening in the length in acceptable ranges. In cases with a major gap of more than 2mm, the union time was shortened after dynamization. The rate of early callus formation was a little higher in cases with postoperative controlled weight bearing than those to whom we did not permit a controlled weight bearing.