Objectives : The purpose of this study was to estimate the antioxidant activity of Sesami Semen Nigrum extract (SSN) on mouse Leydig cells, TM3. Methods : Cell viability assays were performed. The protective effects of SSN against hydrogen peroxide-induced oxidative stress in Leydig cells were examined by measuring cell viability. Lipid peroxidation levels and antioxidant enzyme concentrations such as SOD and catalase were measured. Results : Cell viability of Leydig cells increased with SSN concentration. Cell viability of Leydig cells was 136.66% when SSN concentration was $50{mu}g/ml$. Cell viability of the hydrogen peroxide group was statistically decreased (p<0.01) compared with the control group. Antioxidant effect of SSN was measured and the protective effect of SSN concentration were 5, 10, $50{mu}g/ml$. LPO were decreased significantly at 5, $50{mu}g/ml$ of SSN concentrations. SOD activity was increased at 1, 10, $50{mu}g/ml$ of SSN concentrations. Catalase activity was significantly increased at 123.7, 133.3 and 131.9 units/mg protein when SSN concentrations were 5, 10 and $50{mu}g/ml$, respectively. Conclusions : In conclusion, Sesami Semen Nigrum extract has antioxidant activities in Leydig cells against oxidative stress.