Diabetes mellitus is a common chronic disease affecting millions of people world wide. Standard treatment is failing to achieve required correction of blood glucose in many patients. Therefore, there is a need for investigating potential hypoglycemic drugs or herbs to improve glycemic control in diabetic patients. Nigella sativa seeds were used as an adjuvant therapy in patients with diabetes mellitus type 2 added to their anti-diabetic medications. A total of 94 patient were recruited and divided randomly into three dose groups. Capsules containing Nigella sativa were administered orally in a dose of 1, 2 and 3 gm/day for three months. The effect of Nigella sativa on the glycemic control was assessed through measurement of fasting blood glucose (FBG), blood glucose level 2 hours postprandially (2 hPG), and glycosylated hemoglobin (HbA1c). Serum C-peptide and changes in body weight were also measured. Insulin resistance and beta-cell function were calculated usin the homeostatic model assessment (HOMA2). Nigella sativa at a dose of 2 gm/day caused significant reductions in FBG, 2hPG, and HbA1 without significant change in body weight. Fasting blood glucose was reduced by an average of 45, 62 and 56 mg/dl at 4, 8 and 12 weeks respectively. HbAlC was reduced by 1.52% at the end of the 12 weeks of treatment (P<0.0001). Insulin resistance calculated by HOMA2 was reduced significantly (P<0.01), while B-cell function was increased (P<0.02) at 12 weeks of treatment. The use of Nigella sativa in a dose of 1 gm/day also showed trends in improvement in all the measured parameters but it was not statistically significant from the baseline. However, no further increment in the beneficial response was observed with the 3 gm/day dose. The three doses of Nigella sativa used in the study did not adversely affect either renal functions or hepatic functions of the diabetic patients throughout the study period. In Conclusion: the results of this study indicate that a dose of 2 gm/ day of Nigella sativa might be a beneficial adjuvant to oral hypoglycemic agents in type 2 diabetic patients.