The fractures of tibial shaft are common and open injury are also frequenly accompanied. Since the AO unreamed interlocking nail is solid type compared to present tubular type nail, it has no dead space, and without reaming the intramedullary blood supply can be preserved and the soft tissue trauma can be minimized. Therefore it can be used with minor risk of infection in the treatment of open tibial shaft fractures without secondary operation such as in the cases using external fixaters. Especially in Orientals, whose tibial medullary canal is narrower than Smm, it can be used ideally. From October 1991, we have experienced 14 cases of tibial shaft fractures treated with AO unreamed interlocking nail. Among 14 cases, the open fractures were 8, 4 cases were type 1 and 4 cases were type 2 by Gustilo classification. Among 6 cases of closed fractures, 4 cases had risk of post operative infection due to accompanying soft tissue injuries, such as abrasion or bulla and 2 cases had narrow intramedullary canal less than 8mm. There was no postoperative infection in all cases including open fractures, and the soft tissue defect could be reconstruted by muscle transfer, etc. The AO unreamed interlocking nail is thought to be recommendable in the treatment of open tibial shaft fracture, closed fracture with risk of postoperative infection, and also the fracture with narrow canal diameter less than 8mm.