PURPOSE: This study was performed to evaluate the results of treatment of the humerus shaft fracture with interlocking IM nail. MATERIALS AND METHODS: The result of IM nailing for 29 patients with humerus shaft fracture were evaluated radiologically and clinically. We tried to compare the result of Seidel nail and that of interlocking nail. RESULTS: The results were as follows ; 1. Nonunion rate was 50% in Seidel nail group and 27% in interlocking nail group(p=0.264). 2. Operation time was shorter in Seidel nail group(67.8min) than in interlocking nail group(115.4min)(p=0.002). 3. Fracture site distraction was present in 9 cases and resulted in nonunion in 8 cases. 4. Union time except the nonunion cases was 10.4weeks in Seidel nail group and 11.9weeks in interlocking nail group but was not considered to be significant due to many nonunion cases. 5. Proximal protrusion was present in 6 cases but resulted in shoulder LOM and pain in only 2 ). cases. CONCLUSION: Although the operation time in Seidel nail group was shorter than in interlocking nail group, we couldn't find any other difference between the two nails. The results of treatment of humeral shaft fracture with interlocking nail was unsatisfactory in terms of union rate, complications, union time and functional result.