This study examined the antidiabetic and antihyperlipidemic activities of an ascidian (Halocynthia roretzi) in rats in vivo. Rats were fed on experimental diet including dried ascidian powder (200 mg/kg body weight) for 4 weeks, and then the triglyceride and total cholesterol levels in blood were analyzed. On the ascidian tunic powder diet, the triglyceride level decreased by more than 20.9% and the total cholesterol content decreased by more than 24.4%. In comparison, the triglyceride and total cholesterol level in the blood of rats fed ascidian meat powder decreased only slightly. Therefore, the ascidian tunic powder might be a healthy food with antidiabetic and antihyperlipidemic effects.