The optimal femoral cancellous bone angle is estimated in order to design the femoral IM (Intra-Medullary) nail for patients with more precise method based on CT image of the femur by converting the CT image files into the JPEG files and analyzing the mathematical models with MATLAB (Ver. 6.0). The results show that the measured average ℓ(femur length between Lesser trochanter and greater trochanter) of the right femoral cancellous bones out of 40 Korean males is 356.6 mm whereas the measured average ℓof the Caucasians is 430 mm and the estimated lateral bowing angle( of Korean males is 4.53°±0.93° whereas the estimated lateral bowing angle of the Caucasians is 4.2°±2.3°. Moreover, we have found that the longer femur is bended more than the shorter one and the distribution of the femoral bone length is not the Gaussian profile. This suggested method has a potential to estimate each patient’s femoral cancellous bowing angle more precisely. If the bowing angle of the lateral images of each patient can be estimated immediately through medical image data, it could be easier for surgeons to choose appropriate nails that are most suitable for each patient. More investigations can be achieved by collecting more data for patient’s femoral curvature.