Femoral shaft fractures result from high energy trauma such as traffic accident or falls, and it is frequently accompanied by multiple fractures and several other organ injuries. Intramedullary nail is the effective device available for patient with appropriate fracture of the femoral shaft, and it may also prevent several complications, such as infections and delayed union that can be caused by periosteum and soft tissue injury. Rigid internal fixaton of the femoral shaft fracture with interlocking intramedullary nails presents a significant advance in the management of unst-able fractures and provides longitudinal and rotational stability as well as early joint motion and weight bearing. This procedure has been used with wide popularity because it is possible simultaneously to preserve the range of motion and to obtain the bone union. The purpose of this sutdy is to evaluate the differences of the functional results and duration of the bone union based on Winquist-Hansen classification. Also we analyzed the causes of the complications and results of the treatment. Forty one cases of the femoral shaft fractures were treated with interlocking intramedullary nailing during the period from January 1993 to June 1995 over 12 months follow up. The results were as follows: 1. Among 41 cases of the femoral shaft fractures, static nailings were used in 34 cases and 7 cases with dynamic nailings, then bone union was achieved at 18 weeks for static mode and 15 weeks for dynamic mode respectively. 2. Intraoperative complications were new fractures near the original fracture site(2 cases) and femur neck(1 case). 3. Postoperative complications were delayed union(4 cases), metal failre(1 case), limb shortening(1 case) and distal screw breakage(1 case). 4. Delayed union developed in 4 cases with static mode, but bone union was achieved at average of 22 weeks after changing to dynamic mode at 15 weeks. 5. Functional results were assessed by Karlstrom and Olerud criteria and 80% of the patients were in excellent and goood results.