The aim of this study was to evaluate the possible protective effects of the volatile oil of Nigella sativa (NS) seeds on insulin immunoreactivity and ultrastructural changes of pancreatic beta-cells in STZ-induced diabetic rats. STZ was injected intraperitoneally at a single dose of 50 mg/kg to induce diabetes. The rats in NS treated groups were given NS (0.2 ml/kg) once a day orally for 4 weeks starting 3 days prior to STZ injection. To date, no ultrastructural changes of pancreatic beta-cells in STZ induced diabetic rats by NS treatment have been reported. Islet cell degeneration and weak insulin immunohistochemical staining was observed in rats with STZ-induced diabetes. Increased intensity of staining for insulin, and preservation of beta-cell numbers were apparent in the NS-treated diabetic rats. The protective effect of NS on STZ-diabetic rats was evident by a moderate increase in the lowered secretory vesicles with granules and also slight destruction with loss of cristae within the mitochondria of beta-cell when compared to control rats. These findings suggest that NS treatment exerts a therapeutic protective effect in diabetes by decreasing morphological changes and preserving pancreatic beta-cell integrity. Consequently, NS may be clinically useful for protecting beta-cells against oxidative stress.