Objective To determine the effect of interlocking intramedullary nail in treatment of open tibial and fibula fractures and analyze the method to promote the bone union. Methods From December 2003 to June 2006, thirtyfive patients with open tibial and fibula fracture were treated with emergency debridement, interlocked intramedullary fixation for tibia and fixed fibula at the same time. During operation, the bone marrow was collected and grafted into the fracture gaps. Among them, there were 27 males and 8 females, involving in 22 left legs and 13 right legs. Their ages ranged from 19 to 65 years, with an average of 34.7 years. The location of fracture was the middle of the tibia and fibula in 16 cases, the distal 1/3 of the tibia and fibula in 12 cases and the proximal 1/3 in 7 cases. According to the Gusitilo classification of open injuries, there were 7 cases of type I, 19 cases of type II, 8 cases of type III a and 1 case of type III b. The mean range of knee motion was 48.3° (45-70°). The mean time from injury to operation was 4.3 hours (50 minutes to 7 hours). Results The mean operation time was 94 minutes (60-132 minutes) and the mean blood loss was 122 mL (100-350 mL). The wound healed by first intention in 32 patients. Incision was sutured in 2 cases of type III a fractures after operation 4 days, gastrocnemius flap graft was performed in 1 case of type III b fracture 1 week after operation. They all achieve good heal ing. No fractures spl it off, no iatrogenic nerve and vascular injury occurred, no osteofascial compartment syndromes or deep vein thrombus happened. Tension bl isters appeared in 1 case of type II fracture after operation and subsided after 5 days. Patients were followed up for 14-43 months (mean 22 months). The X-ray films showed that fracture union was observe in 30 cases after 14 weeks of operation, in 3 cases after 18 weeks and in 1 caseafter 22 weeks of operation. The fractures union time was 15.2 weeks on average. About 2 cm nonunion in lateral tibial appeared in 1 case of type I fracture. No fracture occurred again. The mean range of knee motion was 127° (121-135°). The mean HSS score was 96.5 (87-100) at the end of the follow-up. The excellent and good rate was 100%. Conclusion The curative effect of interlocking intramedullary nail is definite in treatment of open tibial and fibula fractures and it can enhance fracture union to plant bone marrow into the fracture gaps.