We had investigated the effect of dynamization in static interlocking intramedullary nailing for the long bone fracture of the femur and tibia treated in Bongseng Memorial Hospital for last 3 years (from Jan. 1994 to Jan. 1997) Total 62 patients (37 tibia, 28 femur) were treated, 35 cases were reamed at operaion and 27 cases not reamed. following results were obtained; 1. In all except 24 fractures, the static interlocking intramedullary nail was preserved without dynamization and got to mean union time of 17.2 weeks (femur) and 6.4 weeks (tibia). 2. The 24 patients (14 femur, 10 tibia) did not show callus formation and complained of vague pain in fracture site at 20 weeks after static mode. In those cases, we tried dynamization as a initial treatment modality. 3. The success rate after dynamization was about 87%. 4. The interval between nailing and dynamization did not affect the success rate. 5. Dynamization could be done day surgery.