The study was performed to explore the suppressive effect of Jew's mellow leaves (JML) on postprandial blood glucose levels in rats and humans. A soluble dietary fiber (SDF) was extracted from the freeze-dried JML powder. An elevation of the postprandial blood glucose level in rats given 1% or 2% JML-SDF solution orally together with 20% glucose solution was significantly suppressed as compared with that observed in the control rats given only glucose solution. When seven healthy young male adults ingested 225 mL of JML mixed juice containing 15 g of freeze-dried powder with 75 g of glucose in the fasting state in the morning, the elevation of the postprandial blood glucose level was significantly suppressed as compared with the control subjects. The diffusion rate of glucose and the permeation rate of glucose in the cultured Caco-2 cells were both significantly reduced by the addition of appropriate amounts of JML-SDF when compared to the controls. These results indicate that the effective substance in JML for suppressing blood glucose elevation is a kind of mucilaginous SDF. The mechanism by which this suppression occurs may be largely attributable to the delayed absorption of glucose from the intestinal membrane in the upper digestive tract by viscous SDF.