A 3D-FEM model was developed to analyze a failure problem of nail used for fixation of humerus fracture. Stress concentration was analyzed under bending and torsion, and it was found that the maximum stress concentration took place around the circular hole that was the actual fracture point of the broken nail. It was also found that bending deformation mode gave more severe stress concentration than torsional mode. It was thus thought that the fracture of the nail occurred under bending condition during hard movement of injured humerus.