We evaluated the role of Tribulus terrestris in males with unexplained infertility and its effect on serum testosterone and semen parameters. Thirty randomized male patients presenting to Andrology outpatient clinic complaining of idiopathic infertility were selected. They were given Tribulus terrestris (750 mg) in three divided doses for three months. The effect of Tribulus terrestris on serum testosterone (total and free) and luteinizing hormone (LH), as well as its impact on semen parameters in those patients, was studied. No statistically significant difference was observed in the levels of testosterone (total and free) and LH and semen parameters (sperm concentration or motility, or abnormal forms) before and after the treatment. In addition, no statistically significant correlations were observed between testosterone (free and total) and LH and semen parameters before and after the treatment. Tribulus terrestris was ineffective in the treatment of idiopathic infertility.