The present study was conducted to evaluate the effect of KST221085, a newly synthesized antidiabetic agent, on the hearts from streptozotocin (STZ)-induced diabetic rats. In isolated diabetic hearts, left ventricular developed pressure (LVDP), heart rate (HR) and coronary flow rate (CFR) were decreased compared to normal control, indicating cardiovascular dysfunction in diabetic heart. The treatment with 10 $mu$M KST221085 remarkably improved the diabetes-induced contractile impairment, without any influence on HR. Reduced coronary flow in diabetic heart was also significantly increased by treatment with 10 $mu$M KST221085. In isolated aorta from diabetic rat, treatment with 10 $mu$M KST221085 increased endothelium-dependent relaxation, suggesting that KST221085 can improve the impaired endothelial function in diabetic aorta. Our results suggest that KST221085 treatment can improve the cardiovascular dysfunction in STZ-induced diabetic rats.