Objective To investigate the protective effect of testosterone on motor neuron after sciatic nerve injury. Methods Twelve adult C57 male mice were randomly divided into two groups: the sesame oil control group (n= 6) and the testosterone experimental group (n= 6). Unilateral sciatic nerve cutting severed as the animal model. Following operation, sesame oil or testosterone was administered for the control and experimental animals respectively via subcutaneous injection every other day for two weeks. The number and soma area of the anterior horn motor neurons in the lumbosacral cord of the sciatic nerve injury side were then measured on Nissl stained sections. Results In the testosterone group the motor neurons appeared healthier than that in the sesame control group. The neurons were plumper and showed more processes. Their number and average soma size were significantly larger than that in the sesame control (P<0.01). Conclusion The findings demonstrate that testosterone has a significant protective effect on sciatic nerve motor neurons after nerve injury.