Interlocking intramedullary nailing has become a common method of treatment for femoral shaft fractures in adults. But sometimes the malalignment of the femoral fracture results in pain, limb length discrepancy, and traumatic osteoarthritis of the knee, etc. Therefore, it is very important to know what makes the malalignment after the femoral shaft fractures. We performed CT scan in 46 patients who had femoral shaft fractures, treated by interlocking intramedullary nailing at the orthopaedic department of the Kwanaiu Verterans Hospital. There were thirty-five men and eleven women, and their mean age was 36 years at the time of the operation. We measured the rotational deformity of both femurs by the CT scanning and the angulation deformity by plain radiographs in forty-six patients. We also compared the amount of the angulation and rotational deformity according to the type or the site of fracture, the degree of comminution, the time from injury to operation and the associated injury. Average angulation deformity was 2.7degrees in sagittal plane and 2.5degrees in coronal plane. Average rotational deformity wat 10.2degrees Fourteen patients(30%) had angulation more than 5degrees Ten patients(22%) had rotational deformity more than 15degrees Angulation deformity was severe significantiy at proximal 1/3 fracture, segmental fracture and severely comminuted fracture group. But because there was no significant difference of rotational deformity according to the level of fracture, the amount of comminution, and associated injury, increased rotational deformity seems to be resulted from the preoperative traction and the intraoperative technique. Therefore, we must determine the accurate entry point of intrameduiiary nailing and reduce the fracture accurately by intraoperative ultrasonography or fluoroscopy.