Interlocking nailing technique has many advantages in the fracture treatment. Such as the prevention of infection and devitalization by closed technique, stable intramedullary osteosynthesis, bone grafting effect of reaming debris, and early restoration of loading and function, comparing to the conventional intramedullary nailing. However, we met many problems during the operative procedure due to deficient concept and technique. We have checked the possible problems during and after the procedure of interlocking nailing for the tibia or femur fractures in 69 cases. 1. Sixty nine cases were consisted by 26 cases of femur fractures and 43 cases of tibia fractures. 2. The inadequate implant length was the most common error in the implant selection (4 cases in femur, 6 cases in tibia). 3. A new fragment was made in 12 patients(17%) during surgery, more commonly in the femur (9 cases), especially medial side and distal to the fracture line. 4. Bone shortening, angulation deformity or rotational deformity, caused by incomplete reduction, were developed more commonly in the tibia than in the femur. 5. Post-operative deep infection were developed in 2 cases(3%). 6. Delayed or non-union were developed in 18 cases(28%), who were followed for more than 3 months. 7. Circumferential wiring or bone grafting using semiclosed technique is thought to be useful to prevent delayed or nonunion when there is a wide gap between fractured fragments pre-operatively.