Background: Cadmium, as an environmental pollutant, exerts its destructive effects on male reproductive system and sperm by inducing oxidative stress. Silymarin, an effective substance extracted from silybum marianum, is a potent antioxidant which inhibits oxidative stress. Therefore, the use of antioxidants, especially natural antioxidants, with the aim of removing free radicals, as well as strengthening the antioxidant defense system can be considered as an appropriate strategy to minimize of oxidative stress-effects to prevent infertility. The aim of this study was to investigate if silymarin can protect the harmful effects of cadmium chloride on human sperm motility, viability, total antioxidant capacity and lipid peroxidation. Materials and Methods: In this study, high quality spermatozoa were used. The samples were washed by human tubal fluid containing bovine serum albumin. The sperm suspensions were divided into 5 groups (2×107 spermatoza per group). 1. Spermatozoa at 0 hour 2. Spermatozoa incubated for 180 minutes (control) 3. Spermatozoa treated with cadmium chloride (20mM) for 180 minutes 4. Spermatozoa treated with silymerin (2mM) for 180 minutes 5. Spermatozoa treated with silymarin+cadmium chloride for 3 hours. Sperm motility was performed according to World Health Organization (WHO) guidelines and eosin nigrosin staining, while malondialdehyde (MAD) and ferric reducing antioxidant power (FRAP) were assessed to investigate lipid peroxidation and total antioxidant capacity respectively. The results were analyzed using one-way ANOVA and P<0.05 was considered significant. Results: Cadmium chloride reduced a significant (P<0.000) in percentage of motility, viability and total antioxidant capacity, while increased (P<0.001) the amount of MAD compared to the control group. In the silymerin+cadmium chloride group, silymarin could significantly (P<0.001) reverses the toxic effect of cadmium chloride on these parameters, when compared to the cadmium chloride gr
