In the treatment of femoral shaft fracture, closed interlocking medullary nailing become popular treatment modality recently even in the case of severly comminuted fracture including bone loss and any site in the femoral shaft. Intramedullary nail acts as a internal splint of the fractured shaft and have load sharing effect in contrast to plate fixation. So it should have proper biomechanical property that is flexible for easy nailing and good contact, rigid and well tolerable to stress. In this particular study, we did stress analysis of differences between anteriorly placed slot and posteriorly placed slot. Slot of the nail make the nail more flexible and more tight fitting. The purpose of this study is to evaluate biomechanical stress changes according to the position of the slot of the nail and the amount of load sharing at the fracture site. The method of study is based on the analysis of experimental nail model using finite element method which commonly used in computer aided engineering. The results are (1) The stress distribution is relatively low in the absence of slot and the difference in more clear in torsional force. But not significant differences in applying the axial loading and bending moment. It seemes to be negligible the difference of rigidity the. benefit of no slot as considering the yielding strength of nail (2) There is no differences in stress distribution between the anteriorly placed slot and posteriorly placed slot. Same findings are resulted on femur-nail-bolt model. (3) We calcaulate the proportion of the load sharing on fracture site in femur-nail-bolt model, the result is about 64%, In conclusion, ideal intramedullary nail should have slot and it is not the significant problem whether the slot is placed on anterior side or posterior side of nail.