PURPOSE: An analysis of primary causes and treatment of the nonunion of humeral shaft fracture. MATERIALS AND METHODS: Incidence of nonunion of humeral shaft fracture among the 183 cases was 10.4%. Among 19 cases of nonunion of humeral shaft fracture from March 1994 to December 1999, 14 cases were men and 11 cases were in third to fifth decade. The causes of the fracture were mainly due to motor vehicle accident and machinery injury. The most common site of nonunion in humeral shaft was at middle 1/3 in 10 cases. RESULTS: Underlying causes of nonunion were complex ; 9 cases of inadequate plate internal fixation, 2 cases of infection, 6 cases of poor external immobilization and 7 cases of over distraction and soft tissue interposition. Bony union was obtained by compression plate, intra medullary nail, and external fixator with autogenous bone graft in 3.5months. Post-operative complications were radial nerve palsy in 4 cases and elbow and shoulder joint contracture in 6 cases. CONCLUSIONS: The main factors that influence the development of humeral nonunions were over distraction of the fracture, inadequate internal fixation, and infection. We should consider that enough operative exposure, the proper choice of plate and screw depending on the bone contour and fracture site, adequate period of immobilization, and rigid fixation in internal plate fixation. It is also recommanded that interposed soft tissue be removed for anatomical reduction. Union was obtained in all cases .