Diabetes Mellitus (DM) is the most widespread endocrine disease characterized by hyperglycemia, and impaired carbohydrate, lipid, and protein metabolism. Recently, numerous research focused on the potential application of adult mesenchymal stem cells (MSCs) as an alternative therapy for diabetes treatment owing to the diverse properties of these cells with hopeful results. Thus, the present study aimed to investigate the potential of type 1 diabetes mellitus (T1DM) recovery through the engraftment of undifferentiated MSCs, as well as the impact on the pancreatic β cells. The experiment was carried out on thirty adult male and female albino rats weighing (180-200 gm each). Twenty-five female rats used in the experiments were divided into 3 groups; the control group (5 rats), the diabetic group injected intra-peritoneal at a dose (60 mg/kg, i.p.) streptozotocin (STZ) (10 rats), and the diabetic group treated with 3×10⁶ of undifferentiated male bone marrow MSCs) BM-MSCs) I/V at a single dose (10 rats). BM-MSCs were isolated (from 5 male donor rats), cultured, and characterized by flow cytometry. The pancreatic tissues from the experimental groups were dissected and prepared for histological, histochemical, immunohistochemical, molecular as well as morphometric studies. The obtained results showed that the diabetic untreated animals had vacuolations, reduction in the size of the pancreatic islets, and congestion of the blood vessels. Moreover, a positive PCR product of the Sry gene, a highly significant increase in insulin immunoreactivity, and restoration of the normal architecture of pancreatic islets in the treated diabetic group with BM-MSCs were detected