Allium cepa (onion) has a beneficial effect on disease treatment worldwide and has been used since ancient times as a medicinal and food source. Recently several reports have shown that onion has high antioxidant activity. As antioxidants have an essential effect on sperm health parameters, we investigated the effect of the fresh juice of onion bulbs on the spermatogenesis cycle in rats. Wistar male rats (n = 30) were allocated into 3 groups, control (n = 10) and two test groups (each of 10). The animals in the test groups were subdivided into groups of 2 that received fresh onion juice equivalent to 0.5 and 1 g/rat/ /day of fresh onion. The fresh onion juice was administered by gavage for 20 consecutive days. The animals were kept in standard conditions. On the twentieth day, the testes of rats in all groups were removed and sperm was collected from the epididymis and was prepared for analysis. Serum total testosterone significantly increased in all the test groups (p < 0.05), and levels of LH significantly increased only in the group that received the high dose of fresh onion juice (p < 0.05), but the level of FSH did not differ between the experimental and control groups. The percentage of sperm viability and motility in both test groups significantly increased (p < 0.05), but the sperm concentration significantly increased only in the group that received the high dose of freshly extracted onion juice (p < 0.05). It was evident that there was no difference on sperm morphology and testis weight in test groups compared to the control group. In our study, freshly prepared onion juice significantly affected the sperm number, percentage of viability, and motility; it seems that using 4 g/kg of freshly prepared onion juice is effective in sperm health parameters.