In the operative treatment of a long bone fracture, the most important point is to secure the bone healing and to strengthen the bone after removal of internal fixation materials. In this regards, the author has analyzed the mechanical strength of various internal fixation methods (a plate, a medullary nail, screws and encircling wires) on the bone by using acrylics models. Experimental studies of their influences on the bone have also been performed especially investigating the decrease in the bone strength in rabbits. All of internal fixation materials are made of 316L stainless steel. Recently, there has been a tendency to use larger materials for internal fixations such as an AO compression plate or a Küntscher type medullary nail. However, it was found that the rigid plate fixation greatly affects the bone strength. A medullary nail produced less impairment to the bone. But this method is restricted in its application because of the weak fixability of the torsional load. On the other hand, the internal fixations with screws or encircling wires are weaker than the other methods especially against the bending load. Therefore the commencement of postoperative treatment will be delayed. But, these two methods have little influence on the bone during fixation.