Abstract: Aqueous extract of leaves of 3 herbs (Murraya koenigii, MK; Psidium guajava, PG and Catharanthus roseus, CR) were used to test their antidiabetic activity in Streptozotocin (STZ) induced diabetic albino rats. MK, PG and CR are given to the STZ induced diabetic rats at the concentration of 500 mg/kg body weight in different groups of 6 diabetic rats each orally once a day for 15 days. Glibenclamide (GBC) is also given to another group to support the results at the concentration of 3 mg/kg body weight orally once a day for 15 days. Diabetic control received vehicle. Body weight showed significant increase (MK and PG: p<0.05, CR and GBC: p<0.001) after 15 days of treatment with herbal extract when compared with the control. Blood glucose level on 15th day of treatment become significantly low (p<0.001). At the termination of the experiment (on 15th day) the urine glucose and ketone were absent in herbal treated group which was present in the diabetic control. Histological study of the pancreas also assesses the results of body weight and blood glucose level. Islets of diabetic control group were damaged, shrunken in size and infiltration of lymphocytes was observed. While islets of herbal extracts treated rats were comparable to normal rats. Many rounds and elongated islets were evenly distributed through out the cytoplasm. No significant histological alteration was found in glomeruli or any other segment of kidney tubule in STZ induced diabetic rats. In herbal extract treated group no difference was found in kidney tubules when compare with their respective diabetic control. Findings of the present study suggest that the aqueous extract of leaves of MK, PG and CR at the dose of 500 mg/kg body weight brings about significant beneficial effects in various physiological/ histological parameters altered during diabetic manifestations and these effects are quite comparable with glibenclamide (a standard drug used to treat diabetes mellitus).