The aim of the present study is to evaluate the effect of hot water extract of black tea in regenerating beta cells in streptozotocin-induced diabetic mice. Light microscopic examination of pancreatic sections of streptozotocin-induced diabetic mice showed the acinar cells to be small, shrunken, and with deteriorated beta cells. The dose of streptozotocin not only altered the function of beta cells but also damaged the acinar region. The changes in acinar cells were coarsening of endoplasmic reticulation suggesting alteration in their secretory function. The control pancreatic tissue showed well-defined granulated islets and dark beta cells when stained with chrome hematoxylin and phloxine. Interestingly, pancreatic sections of diabetic mice fed with black-tea extract showed regeneration of beta cells and acinar region appeared normal with increased numbers of beta cells. To understand the probable mechanism of action of black-tea extract, we analyzed inducible nitric oxide synthase (iNOS) expression by immunohistochemistry and the results showed an increased iNOS levels in streptozotocin-induced diabetic pancreas, and such high iNOS levels were inhibited in black-tea extract treated mice. According to histological results obtained, it can be concluded that the black-tea extract helps in regeneration of damaged pancreas and protects pancreatic beta cells by its antioxidant action against nitrosative stress in streptozotocin-induced diabetes.