The antidiabetic effects of corosolic acid (CA) were investigated in KK-Ay mice, an animal model of type 2 diabetes. CA (2 mg/kg body weight) reduced the blood glucose levels of KK-Ay mice 4 h after a single oral dose. CA (2 mg/kg) reduced the blood glucose levels in KK-Ay mice 2 weeks after a single oral dose and also significantly lowered plasma insulin levels were in KK-Ay mice under similar conditions. CA-treated KK-Ay mouse blood glucose significantly decreased in an insulin tolerance test. These results support the hypothesis that CA improves glucose metabolism by reducing insulin resistance. Therefore CA may be useful for the treatment of type 2 diabetes.