PURPOSE: The purpose of this study was to analyze the factors influencing the failure of union in the intertrochanteric femoral fractures. MATERIALS AND METHODS: Authors divided the patients treated with lag screws into two groups, 9 cases of the non-union group and 46 cases of the union group. And analyzed the preoperative, intraoperative and postoperative factors influencing the union. RESULTS: Age, displacement in AP film after surgery, displacement of lesser trochanter after surgery, superior or anterior position of lag screw, displacement of lag screw and change of neck-shaft angle were significantly higher in the non-union group. Displacement in AP radiograph after surgery and displacement of lag screw were two significant factors by multivariate logistic regression analysis. CONCLUSION: In case of intertrochanteric fracture treated with lag screw, the probabilities of union failure are increased when the displacement is more than 5 mm in AP plane postoperatively and the sliding of lag screw is more than 10 mm in follow-up.