Objectives : The purpose of this study is to examine the antioxidant effects on male mouse reproductive cells of the extract of Platycladi Semen. Methods : The extract was studied for diphenyl-picryl-hydrazyl (DPPH) radical scavenging activity, cell viability by a modified MTT assay, the effects on $H_2O_2$-induced cytotoxicity by MTT assay, lipid peroxidation by malondialdehyde (MDA) formation and super oxide dismutase (SOD), respectively. Results : The results showed that the extract scavenged DPPH radical in a dose-dependent manner by up to 74.87%. The cell viability of the extract was within 72~96% on Leydig cells and GC-2 cells at concentrations of 1, 5, 10, 50 and 100 ug/ml. The hydrogen peroxide-induced cytotoxicity of Leydig cells was protected to 72.09% by the extract at concentration of 100 ${mu}g/ml$. The hydrogen peroxide-induced lipid peroxidation of MDA formation was decreased to 1.80 and 1.65 nmoles/mg protein by the extract at concentrations of 50 and 100 ${mu}g/ml$. The extract at all concentrations, SOD activity was not significantly changed. Conclusions : In conclusion, the extract of Platycladi Semen has antioxidant effects on Leydig cells and protect male reproductive system against oxidative stress.