PURPOSE: To analyze clinical and mechanical factors of the fatigue fracture of the intramedullary nail in the treatment of the femoral shaft fractures and to consider preventive methods of fatigue fracture. MATERIALS AND METHODS: We reviewed 12 patients of fatigue fractures of the intramedullary and were followed for a minimum one year. The site of fatigue fracture of the intramedullary nail was at fracture site in 10 cases, just proximal to proximal locking hole in one, and the most proximal of two distal locking holes in one. We analyzed type and diameter of broken nail, time from injury to fatigue fracture, causes of metal failure, and treatment results. RESULTS: Intramedullary nails which had fatigue fracture were reamed AO nail in four cases, Grosse-Kempf nail in four, Russel-Taylor nail in three, and long Gamma nail in one. Time to fracture of implant was average 13.6 months (range, 6~30 months). All cases were treated by intramedullary nailing, and additional autogenous bone grafting was done in three cases. At an average duration of follow-up of 7.5 months (range, 5 to 10 months), all of the fractures had healed. CONCLUSION: To prevent fatigue fracture of intramedullary nail, closed obervation for bony union, progressive weight bearing, and augmentation with autogenous bone grafting for comminuted fracture site should be needed.