Abstract: Senna siamea is traditionally used for treatment of many diseases. This study was carried out to evaluate the antihyperglycemic and antihyperglycemic effect of the methanol extracts of the leaves and stem bark of S. siamea. The extracts were orally administered daily at 250 and 500 mg kg-1 b.wt. for three weeks. The effect of the extracts on Fasting Blood Glucose (FBG) level and body weight were determined weekly. All rats in the diabetic groups had FBG levels well within the diabetic range (>150 mg dL-1) at the initial stage of the experiment but after three weeks of treatment with extracts or glibenclamide; the FBG significantly (p<0.05) dropped in dose-dependent manner, with leaves being more active. So also, the increase in triglycerides, LDL-cholesterol and total cholesterol in diabetic rats were significantly (p<0.05) prevented by the two extracts, with concomitant increase in serum HDL-cholesterol. The increase in serum total protein and decrease in urea in all treated groups was statistically (p<0.05) significant when compared with the diabetic control group. The slight increase in the levels of ALT and AST recorded in the diabetic control group were prevented in the extracts treated groups, though the decrease did not differ significant. These effects were quite comparable with the glibenclamide treated group which is a standard drug used to treat diabetes. The results suggest that the methanol extracts of the leaves and stem bark of S. siamea restored the metabolic changes in alloxan-induced diabetic rats.