We performed a retrospective study of nine humeral shaft fractures which were treated by interlocking intramedullary nailing, the AO UHN (Unreamed Humeral Nail) system between March 1996 and February 1997 with more than one year of follow up. AO UHN inserted by either antegrade or retrograde technics through limited incisions followed by insertion of 2 proximal and distal Interlocking screws. Compression between fractured fragments was achieved in the non-comminuted and indicated cases. Immediate postoperately, soft shoulder immobilizer was applied and rehabilitation was started with active shoulder motion exercise few days to 1 week postoperately as soon as patient could tolerate pain. Union occurred at average of 13 weeks except one expired case with pathologic fracture due to advanced metastatic cancer. Pain relief and functional restoration were rated as good to excellent in most cases. Interlocking intramedullary nailing using AO UHN for the humeral shaft fractures usually provides immediate stability of the fracture and can be accomplished with a closed technique, minimum morbidity, with a resultant early return of function of the extremity. Therefore we recommend AO UHN for the treatment of the humeral shaft fractures if available without hesitation.