The mechanism of ROS' on mitochondrial damage of testes in rats with varicocele was studied. Adult male Wistar rats were randomly divided into 2 groups i.e, varicocele group (VG) and sham operation group (SOG). After three months, bilateral testes were removed to make ultrathin section and their mitochondria were isolated. Index determination was according to the direction of kit. Mitochondrial MDA content of bilateral testes in VG were higher than those in SOG (P<0.01, P<0.05), Na+-K+ATPase, Ca2+-ATPase and GSH-Px activity were lower in VG than those in SOG (P < 0.01, P < 0.05). Result of correlation anal. showed that left testicular mitochondrial MDA content in VG was neg. correlated with Na+K+-ATPase, Ca2+ATPase, GSH-Px activity resp. (r=- 0.718, r=-0.656, r= _0.636, P < 0.05). Mitochondrial SOD activity between two groups had no significant differences. Different pathol. change of bilateral testes were found in VG, characterized by mitochondrial change, and obvious in left ones by using transmission electron microscope. Increased ROS, disturbance of elimination and changes of mitochondrial construction and function can cause testes function disorder and thus lead to infertility.