The purpose of this paper is to compare the results, complications, advantages and disadvantages of treatment by flexible intramedullary nail with those by interlocked intramedullary nail in humeral shaft fractures. The authors analyzed 42 cases of humeral shaft fractures in patients who were treated by intramedullary nailing. Nineteen patients were treated with flexible I-M nail and twenty three patients were treated with interlocked I-M nail. Fourteen cases(74%) treated with flexible I-M nail were obtained primary bone union, and twenty one cases(91%) treated with interlocked I-M nail were obtained primary bone union. Five cases treated with flexible I-M nail and 2 cases treated with interlocked I-M nail failed union and the most common cause of nonunion was distraction. Flexible I-M nailing does not solve rotational deformity and could't obtain a high union rate due to distraction. Therefore, we recommend that adequate external support is considered to prevent distraction using the flexible I-M nail. Although we obtained a satisfactory union rate using by Seidel interlocked I-M nail, we must observe the distal spreading screw loosening during the follow up period. In bones with small diameter, sufficient reaming may be necessary, because the distal spreading screw can't spread well.