The aim of the present study was to evaluate the effect of the extract of Allium cepa (Onion) seeds (AC) on morphometric and histology of testis and biochemical parameters in STZ-induced male rats. Forty adult male Wistar rats (2 month old) were allocated into four groups of control, diabetic control, diabetic treated with 200 or 400 mg/kg/day of onion seed extract. Diabetes mellitus was induced using 60 mg/kg body weight of Streptozotocin as a single intraperitoneal injection. The extract was administered by stomach gavage for 28 days. The morphometric and histological structure of the testis, biochemical factors like glucose and testosterone levels were assessed. All analyses were done at the end of the four week study period. Data were compared by using Kruskal Wallis Test, Dunnett T3 and the degree of significance was set at P < 0.05 and P < 0.01. In diabetic+200 rats, the numbers of primary spermatocytes were significantly increased. In diabetic+400 rats, seminiferous tubular diameter was significantly increased and the level of testosterone hormone and testis weight was decreased significantly. In diabetic+200 and 400 rats, the numbers of spermatid, FBS and lumen diameter were significantly increased and the numbers of spermatozoa cells, body weight and volume density (VD) % lumen were decreased. Also, the numbers of spermatid in control diabetic rats was decreased. Our finding indicated that onion seed extract might be useful as a supplementary protective agent against adverse effects of diabetes on reproductive system in diabetic men.KEY WORDS: Onion, Testis, Rat, Streptozotocin, Histopathology.