Interlocking intramedullary nailing has been the first choice treatment of most tibial shaft fractures because its rigidity of fixation allows early ROM and weight bearing. Although most interlocking nailing procedures are performed with closed reduction that preserve periosteal blood supply, delayed union or nonunion is often occurs. so secondary procedures, such as bone graft, dynamization, nail exchange, are necessary to achieve fracture healing. We analyzed 25 cases of delayed union or nonunion from 432 tibial shaft fractures fixed initially with static interlocking intramedullary nailing since January 1990 till January 1996. Overall incidence of delayed union or nonunion is 5.8%(25/432). Secondary procedures included mainly iliac bone graft or dynamization. Average time to achieve bone union after secondary procedure is 6.1 months. The more distracted or displaced fracture ends after interlocking nailing or the more comminuted fracture fragments led to the more delayed bone union. Careful attention to minimize distraction or displacement during interlocking nailing is necesary to decrease the incidence of delayed union or nonunion. And appropriate procedures should be performed when delayed union or nonunion is predicted, which result in success in most cases.