Aim: To evaluate the side effects of Tamsulosin hydrochloride in fertility of experimental rats. Materials and methods: three groups of mice were used. First and second groups were injected [intraperitoneal (I.P.)] daily for 42 with 8 and 16 µg /kg mouse body weight (kg.b.wt) of Tamsulosin hydrochloride, respectively. Third group was injected with PBS (control). Several biological and histopathological studies were conducted on rat groups. Results: Significant decrease in number, motility and viability of epididymal sperm post injection with 16 µg /kg.b.wt, while injection with 8 µg /kg.b.wt reduced significantly, percentage of viability of sperm as compared with the control group. High percentage of abnormal sperm was observed in mice that injected (I.P.) with 8 and 16 µg /kg.b.wt versus control group. The injection with both concentrations (8 and 16 µg /kg.b.wt) of Tamsulosin hydrochloride reduced the levels of testosterone (P <0.05), body weight, testes weight, diameter of seminiferous tubules (DST), diameter of primary spermatocyte (DPS), diameter of spermatids and number of Leydig's cells cluster significantly. However, same concentrations of Tamsulosin hydrochloride increased the interstitial space and number of abnormal Leydig's cells cluster (P<0.05). Necrosis and edema was observed clearly in testes of mice that injected with Tamsulosin hydrochloride. Conclusion: Current study proved clearly, the negative effect of Tamsulosin hydrochloride on sperm activity and number. Moreover, both studied concentrations of Tamsulosin hydrochloride affect negatively on testes structure and testosterone level.