Locked intramedullary nailing has been used as a popular method in treating humeral shaft fracture because of relatively less invasive procedure, good stability and allowing early rehabilitation. However many problems such as rotator cuff injury, iatrogenic fracture, difficult distal locking at operation and painful limitation of shoulder motion due to protrusion of the nail above the greater tuberosity, frequent delayed or nonunion were reported. We reviewed our experience with 26 cases of humeral shaft fractures that were treated by interlocking intramedullary nailing to evaluate the pitfalls and the complications of this technique. the results were as follows. 1. In five cases(19.2%) iatrogenic fractures occured during nail insertion. Three were in the site of inlet and two were in the original fracture site. 2. In four cases(15.4%) painful limitation of shoulder motion was persisted for 2 months. Two were caused by subacromial impingement due to nail protrusion and two were by intraoperative rotator cuff injury. 3. Nonunion occured in two cases(7.7%) which were middle one third transverse fractures and were not fixed with distal locking screws. 4. Intraoperative and postoperative complication rate was 42.3%.