The intramedullary nailing is one of the most available method in the treatment of femoral shaft fracture. Since the development of Küntscher nail in 1940, new designs such as the Grosse-Kempf interlocking femoral nail, Ressell-Taylor Femoral nial, AO/ASIF universal nail, ACE titanium nail became widely used by more applicable level of femoral shaft fracture. But complication of intramedullary nailing was rarely seen. The authors analyzed the clinical and mechanical factors predisposing to failures of intramedullary interlocking nails in 6 cases whom treated the comminuted fracture of the femoral shaft, at the department of orthopaedic surgery, Sun General Hospital from 1991 to 1995 and the following results are obtained. 1. Initial fracture patterns were comminuted in all 6 cases. 2. The implant nails were AO nail in 4 cases, Grosse-Kempf nail in 1 case, Russell-Taylor nail in 1 case(nail diameter was 10mm to 12mm). 3. The failure sites of the interlocking nail were the initial bone fracture site in 2 cases, distal to the proximal screw hole in 1 case, proximal to the distal screw hole in 2 cases, fracture site and distal to the proximal screw hole in 1 case. 4. The average time to failure of implants were 11.5 months. 5. The average times to partical weight bearing were 6.3 weeks. In the analy of above results, by using diameter of nail more than 13mm and delaying weight bearing on the injured limb until there is clear radiographic evidence of early union of the femur fracture, we may reduce the metal failures of intramedullary interlocking nail in comminuted femoral shaft fractures.