The effect of a methanol extract of Rosa damascena Mill. flowers was studied, in comparison to the alpha-glucosidase inhibitor acarbose, in normal and diabetic rats. The inhibition mode of this extract was examined by measuring enzyme activity in different concentrations of substrate for Lineweaver-Burk plot analysis. The results show that Rosa damascena extract has an intensive inhibitory effect on alpha-glucosidase. Its inhibition was found to be noncompetitive. Oral administration of this plant extract (100 to 1000 mg/kg body wt.) significantly decreased blood glucose after maltose loading in normal and diabetic rats in a dose-dependent manner. These results suggest that Rosa damascena might exert an anti-diabetic effect by suppressing carbohydrate absorption from the intestine and can reduce the postprandial glucose level.