We retrospectively reviewed one hundred fifty-two patients who were treated for humeral shaft fractures at Eul-Ji Medical College hospital from Jan 1991 to June 1997. We experienced twenty one cases of nonunion among them after primary internal fixation. The purpose of this study was to evaluate possible causes of nonunion and to find out adequate treatments. The main cause of nonunion was an inadequate fixation (57%). The most common fracture pattern was simple transverse type(22%), and it occurred most frequently at the middle one third area of humeral shaft(22%). For the treatment of nonunion, the plate fixation was done in ten cases, interlocking intramedullary nailing in three cases, bone graft only in six cases, and external fixation using monofixator(Orthofix) in two cases. We obtained complete bony union from all cases, and the average union time was 17.5 weeks. Our study showed that higher incidence of nonunion was found in primary intramedullary nailing cases(19%) than in primary plate fixation cases(4%). In conclusion, close attention should be paid when choose the primary fixation method, and plate fixation and bone graft was considered as one of the useful treatment method of nonunion.