Background and Objective: Lettuce (Lactuca sativa) is one of the long-standing herbs in traditional medicine known with many therapeutic properties. In this research, the effect of lettuce leaf extract on some reproductive traits such as testes weight and primary spermatocyte testosterone, LH and FSH in mice was investigated. Materials and Methods: In this experimental study, 40 male mice were selected and were randomly divided into 5 groups, including a control and 3 treatment and placebo groups. Treatment groups of animals received three intraperitoneal lettuce extract in daily doses of 25, 50 and 100 mg/kg for 20 days. Testosterone, LH and FSH tests implemented via Radioimmunoassay (RIA). The collected data analyzed by ANOVA and Duncan's test. Results: The results showed that the number of secondary spermatocytes (42±5) and concentrations of testosterone (0.088±0.023 ng/dl), LH (0.3±0.02 mIU/dl) and FSH (1±0.12 mIU/dl) significantly reduced at 100 mg/kg dose (P<0.05). Conclusion: Considering the results, it seems that the compounds in lettuce extract can exert a negative effect on male mice reproductive physiology as a result of reduction in spermatocytes and reproductive hormones.