We investigated the antihyperglycemic effect of p-methoxycinnamic acid (p-MCA), a cinnamic acid derivative, on plasma glucose and insulin concentrations, activities of hepatic glucose-regulating enzymes and hepatic glycogen content in normal and streptozotocin (STZ)-induced diabetic rats. p-MCA (10-100 mg/kg, PO) dose-dependently decreased plasma glucose concentration in both normal and diabetic rats in the oral glucose tolerance test. To investigate the chronic effects of p-MCA on glucose metabolism, p-MCA (40 mg/kg, PO) was administered to normal and diabetic rats once a day for 4 weeks. p-MCA reduced plasma glucose concentration in diabetic rats, which was observed during the 4-week study. However, p-MCA treatment did not change plasma glucose concentrations in normal rats during the 4-week study. p-MCA also reduced the excessive activities of hepatic glucose-6-phosphatase, hepatic hexokinase, glucokinase and phosphofructokinase in diabetic rats and increased hepatic glycogen in these rats. In p-MCA-treated normal rats, there were no changes in the activities of hepatic glucose-regulating enzymes, hepatic glycogen and glucose-6-phosphate. Our findings suggested that p-MCA exert its antihyperglycemic effect by increasing insulin secretion and glycolysis, and by decreasing gluconeogenesis.