We studied the technical problems of step-cut osteotomy and a method to improve the problem. When a bony fragment is removed from distal humerus in step cut osteotomy, we obtain a bony defect of the shape of a right triangle. The distal part of this defect is right angled(90 degrees) but the proximal part inserted in it is less 2than 90 degrees, they do not correspond to each other. There is, however, no explanation on this in the original article. By personal communication with DeRosa, he said he would remove a part of the small bony spike of the lateral side after osteotomy. However, it is technically difficult to remove a part of the distal bony spike because it is tiny and it can cause a fracture on the bony spike. Even in a successful 1case, there arises a problem of fixing with a cortical screw. To solve this problem, we instead removed a part of the lateral cortex of proximal part so that we made possible the contact of medullary canal at the osteotomy site without removing the lateral spike of distal bony spike. This way, a more stable fixation and a faster bony union were made possible. This modified method was performed on 17 cases, in which the average age of the patients was 13 years old(9 to 18). For 4-6 weeks after the operation, they were immobilized the upper extremity in a cast, and after that, then were allowed to do exercises. We obtained the complete bony union between the 10th and 16th week after the operation. In the follow-up cases of minimum 1 year, 12 cases were proven excellent, 3 cases were good, and 2 cases were poor by Oppenheim's criteria. There was one case of radial nerve transient paresis and one case of metal failure. There was no nonunion on osteotomy site. By using the modified method, we made possible the mechanically stable and technically easier fixation to obtain satisfactory results.