Purpose:To explore the possible surgical factors related with nonunion in femoral shaft fracture following intramedullary nailing.Methods:We retrospectively analyzed totally 425 patients with femoral shaft fracture in level I urban trauma center,including 254 males and 171 females,with an average age of 37.6 (ranging from 21 to 56)years old.The inclusion criteria included:(1) traumatically closed fracture of femoral shaft,with preoperative films showing non-comminuted fracture,such as transverse fracture,oblique fracture or spiral fracture;(2) closed reduction and fixation with interlocking intramedullary nail at 3-7 days after trauma;(3) complete follow-up data available.The relationship between the following factors (fracture site,reduction degree,direction of nail insertion and nail size) and nonunion was studied.Results:The incidence of femoral nonunion was 2.8％ in patients with closed simple fracture undergoing interlocking intrameduallary nailing,including 11 cases of hypertrophic nonunion.Nonunion was related significantly to distal fracture,unsatisfactory reduction and unreamed nail (p ＜ 0.05).There was no significant difference between antegrade nail and retrograde nail (p ＞ 0.05).Conclusions:Nonunion in femoral shaft facture following interlocking intramedullary nailing is related to fracture site,fracture reduction and nail diameter.The choice of reamed nails or unreamed nails depends on the fracture site and reduction degree.