There are many advantages in treating long bone fractures with interlocking intramedullary nail and its advantages are well known. Although it is used widely, many major and minor complication can develop in spite of meticulous operation techniques. Frist fifty consecutive interlooking intramedullary nail of two different surgeon, total of 100 cases that were operated in Seoul Paik Hospital from January, 1991 to July, 1994, were reviewed and the complications were analyzed with various factors. 1. Among the 100 cases, a total of 30(30%) major and minor complications had developed. But only 4 major complications which needed reoperation or poor result occurred. 2. Among the complications that had developed, angulation and rotation deformity being 10 cases were the most common, other complications were delayed union in 5 cases, nonunion in 2 cases, problem of screw in 4 cases, metal failure in 2 cases, misentry point in 2 cases and others in 5 cases. 3. There were no remarkable difference between surgeon A and surgeon B. The complications decreased abruptly after 30th case for each surgeon and only a few complications, probably due to complex fracture pattern, occurred. In conclusion, complications after interlocking intramedullary nailing can be reduced by meticulous preoperative planning and learning process of surgeon.