Diabetes affects about 4% of the global population and management of diabetes without any side effects is still a challenge to the medical system. The present study investigated the possible protective effects of Costus speciosus (Koen.) sm. (C speciosus) rhizome extracts on biochemical parameters in streptozotocin (STZ)-induced male diabetic Wistar rats. STZ treatment (50 mg/kg, i.p.) caused a hyperglycemic state that led to various physiologic and biochemical alterations. Hexane, ethyl acetate, and methanol crude extracts administered at the dose of 250 mg/kg, 400mg/kg, and 400mg/kg, respectively, for 60 days to STZ-induced hypoglycemic and normoglycemic rats. The plasma glucose concentration was significantly (p <0.05) decreased by all three extracts compared with controls. In addition, oral administration of hexane extract significantly decreased glycosylated hemoglobin (HbA(1c)), serum total cholesterol, and triglyceride levels, urea, uric acid, and creatinine and at the same time markedly increased plasma insulin, tissue glycogen, serum protein, and high-density lipoprotein (HDL) cholesterol levels. The hexane crude extract also restored the altered plasma enzymes aspartate aminotransferase (AST), alanine aminotransferase (ALT), alkaline phosphatase (ALP) and acid phosphatase (ACP) levels to near normal. Glibenclamide used as a reference drug (0.6mg/kg body weight) also produced a significant reduction in the blood glucose concentration in STZ-induced diabetic rats. In summary, the hexane crude extract was found to be more active in comparision with ethyl acetate and methanol extracts. Thus this study shows that the C speciosus hexane extract has antihyperglycemic and hypolipidemic activity, is able to ameliorate the diabetic state, and is probably a source of hypoglycemic compounds.