ABSTRACT: Rosemary (Rosmarinus officinalis) extract with 50% ethanol remarkably inhibited rat intestinal α-glucosidase (sucrase) activity when compared with 31 different herbs and spices (aqueous and 50% ethanol aqueous extracts). Rosemary-distilled extract obtained from 50% ethanol extract by evaporation inhibited α-glucosidase activity in the reaction with both maltose and sucrose. Maltose or sucrose was orally administered, with or without rosemary-distilled extract, to mice at a dose of 20 mg/mouse. A postprandial elevation in plasma glucose levels 30 min after administration of maltose or sucrose plus the distilled extract was significantly suppressed compared with glucose levels in mice that did not receive the distilled extract. A 0.01% aqueous solution of rosemary-distilled extract supplied as drinking water to streptozotocin (STZ)-induced diabetic mice significantly suppressed an increase in plasma glucose levels 4 d after injection of STZ. It was also shown that a 0.01% aqueous solution of the distilled extract inhibited α-glucosidase (maltase and sucrase) in the small intestine of STZ-induced diabetic mice. An active compound with IC50 values of 290 μg/mL (maltase inhibitory activity) and 150 μg/mL (sucrase inhibitory activity) was isolated and identified to be 5,7,3′,4′-tetrahydroxy flavone (luteolin). These results suggested that rosemary extract might be a beneficial food material in the prevention of diabetes and obesity.