【Abstract】 Objective The auto-control micro-motion intramedullary locking nail (AMLN) is designed, to reducethe incidence of delayed union and non-union of femoral shaft fractures fixed by interlocking intramedullary nails, and toobserve the cl inical effect of self-design AMLN in the treatment of femoral shaft fractures. Methods The distal and promixalnails were connected by the micro-motion locking structure, which could cause 1.0-1.5 mm axial micro-motion between fracture gaps. It could produce physical stimulus and conduction between fracture gaps in the course of fracture union. From December 2003 to May 2006, 32 cases of femoral shaft fractures were treated with AMLN, including 21 males and 11 females with the average age of 31.2 years (ranging from 20 years to 43 years). The trauma resulted from fall wounds in 3 cases, crash injuries in 1 case and car accidents in 28 cases. Twenty-nine cases were fresh fractures in different parts of the femoral shaft with transverse, obl ique, spiral and comminuted fractures of type I, II, III and IV. Three cases were old non-union fractures. The fresh fractures were treated by closed AMLN fixation, while the old fractures were treated by open AMLN nails after routine implantation of self bone. Results All the 32 cases were followed up for the average time of 11.5 months (rangeing from 8 months to 22 months). The X-ray films showed the fractures were healed 4.0 to 7.5 months after the operation, with the mean time of 5.1 months, and no break of the nail happened. One nail mildly bent in the comminuted fracture, and 2 patients felt sl ightly unwell at the needl ing point. According to the Klemm criterion for function, 26 cases were excellent, 5 good, 1 fair, and the choiceness rate was 96.88%. Conclusion With a suitable design, AMLN is easy to perform and helpful to quicken fracture union, and it is effective to treat femoral shaft fractures.