The aim of the present study was to examine whether an aqueous extract of Cleistocalyx operculatus flower buds (COB) had protective and anticataract effects on beta-cells in experimental streptozotocin (STZ)-induced diabetes in rats. After 9 weeks of COB supplementation (500 mg/kg bw), the COB group had a significantly more stable insulin level as compared with the control diabetic group. Increased staining of insulin and preservation of islet cells were apparent in the COB-treated diabetic rats, whereas islet cell degeneration and weak insulin immunohistochemical staining were observed in the control diabetic rats. In addition, COB significantly delayed diabetic cataract formation and caused significant reductions in the glucose, sorbitol, and fructose levels in diabetic rat lenses. Furthermore, as compared to the control diabetic group, the COB group also showed antihyperglycemic effects (reductions in plasma glucose and HbA1c levels).