The intramedullary nailing is a good method of treatment in femur shaft fracture but it is not available in some fracture pattern or fracture level. And it is very difficult with conventional Kiintscher nailing to prevent torsional stress and fix rigidly in unstable fracture of the femur shaft. The unstable fracture by comminuted fracture or segmental fracture, nonunion and pathologic fracture needed the rigid fixation. Interlocking Kuntscher IM nailing can provide antitorsional stability, good axial alignment and prevent shortening of the fracture site, also allow early ambulation and joint exercise. We analysed 51 patients 52 cases of interlocking Kiintscher IM nailing from May 1981 to March 1988. The interlocking Kuntscher IM nailing prevents the rotational and axial roading. The interocking Kuntscher IM nailing has expanded its application in fracture pattern and fracture site. The interlocking IM nailing provides rigid fixation in severe comminuted fracture. segmental fracture, long spiral fracture, and other several unstable fractures and eliminates splinting or external supports, so it is possible doing early joint exercise. The interlocking Kuntscher IM nailing was also considered best internal fixation method in pathologic fracture or sever osteoporosis. The static interlocking for rigid fixation and the dynamic interlocking method for axial compression to fracture site during weight bearing can be adapted appropriately to fracture pattern. Radiation hazard during the interlocking nailing is not considerable.