Cinnamomum cassia (C. cassia) has antihyperglycemic properties. This study aimed to assess the hypoglycemic effects of the aqueous extract of C. cassia on the pancreatic tissue of diabetic rats, comparing histological and biochemical outcomes with those of metformin (MET) administration. A total of 42 male albino rats were divided into seven groups. Distilled water was given to healthy rats in the first group, whereas diabetic rats (DRs) induced by alloxan were treated with the same substance in the second and third groups. Rats with diabetes were given C. cassia treatment for 14 and 30 days in the fourth and fifth groups, whereas non-DRs received the same treatment in the sixth and seventh groups. Furthermore, MET was administered to four groups of DRs. Diabetic rats had reduced serum amylase levels and significantly increased blood glucose levels. Histological examination revealed thickening of the basement membrane in the islets of Langerhans blood channels and capillaries, as well as an increase in α- and δ-cell activity and a reduction in β-cell activity. However, administration of C. cassia aqueous extract caused significant alterations in most measured parameters, including increased serum amylase and decreased blood sugar levels. The daily use of C. cassia decreased glucose levels and induced a substantial increase in β-cell activity and a decrease in α-cell activity. Plant extracts have both regenerative and reparative properties.