PURPOSE: To comparative analysis of clincal difference between reamed and unreamed interlocking intramedullary nailing in the treatment of femoral and tibial shaft fractures. MATERIALS AND METHODS: We reviewed femoral and tibial shaft fracture who were treated with reamed or unreamed nail. They were followed for a minimum of 16 months. Winquist-Hansen and Johner-Wruhs criteria were applied for the classification of the fractures. Retrospectively we evaluated the duration of operation, the amount of bleeding, the first time of callus formation, union time, the time of partial weight bearing, isthmic ratio, complications. RESULTS: The average duration of operation for femoral fractures with reamed and unreamed nail were 104 minutes, 95 minutes, respectively. And those for tibial fractures were 96 minutes, 87 minutes, and the difference was statistically significant (p<0.05). The amount of bleeding in femoral fractures with reamed and unreamed nail were 360 ml, 223 ml, respectively. And those in tibial fractures were 280 ml, 205 ml, respectively, and the difference was statistically significant (p<0.001). The isthmic ratio in femoral fracture with reamed and unreamed nail were 105.5%, 87.0%, respectively and those in tibial fracture were 106.3%, 85.3%, respectively. There were 2 delayed unions in femoral fractures and 1 delayed union in tibial fracture with unreamed nail, and 1 metal failure in tibial fracture with unreamed nail. CONCLUSION: Unreamed femoral intramedullary nailing involves fewer steps and less intraoperative blood loss than reamed nailing. There was no statistical difference the first time of callus formation, union time, the time of partial weight bearing. It must be consider that delayed union and metal failure in the unreamed intramedullary nailing due to high grade fracture, lower isthmic ratio, combined with multiple trauma.