The present study was aimed to investigate the effect of black tea extract on blood glucose, plasma insulin, Hemoglobin, carbohydrate metabolic enzymes and tricarboxylic enzymes in streptozotocin (STZ) induced diabetic rats. Diabetes was induced in male albino Wistar rats by intraperitoneal administration of STZ (40 mg/kg b wt). Black tea extract was administered to diabetic rats at a dose of 25, 50 and 100 mg/kg body weight for 30 days. The effects of black tea extract on glucose, insulin and HbA1c levels were analyzed to confirm the effective dose. Administration of black tea extract to diabetic rats was significantly decreased the level of glucose, glycated hemoglobin and increased the levels of insulin in a dose dependent manner. The black tea extracts at a dose of 100 mg/kg b wt showed a highly significant effect compared to other two doses (25 and 50 mg/kg b wt). The effect produced by black tea extract (100 mg/kg b wt) was comparable to that of glibenclamide (5 mg/kg b wt) a reference anti diabetic drug. Therefore, 100 mg/kg b wt was fixed as an effective dose and used for further analyses. Black tea extract was administered to diabetic rats at a dose of 100 mg/kg b wt for 30 days reinstated the altered levels of the plasma glucose, insulin, hemoglobin, glycosylated hemoglobin, carbohydrate metabolizing enzymes and tricarboxylic cycle enzymes in diabetic rats. Black tea extract administered to diabetic rats at a dose of 100 mg/kg b wt for 30 days reinstated the altered levels of the plasma glucose, insulin, hemoglobin, glycosylated hemoglobin, carbohydrate metabolizing enzymes and tricarboxylic cycle enzymes in diabetic rats. The effect produced by black tea extract of all the biochemical parameters were comparable with glibenclamide-used as a reference drug. [ABSTRACT FROM AUTHOR]