Objectives : The root and rhizome of "Gangwhal" have been used as a traditional medicine for the treatment of cold, fever, headache, swelling, arthritis, rhinitis, and cardiovascular diseases in Korea and china. In china, Gangwhal is well known as a useful oriental medicinal plant that treats cardiovascular diseases such as stroke, headache and hypertension, but little research exists about the effect of O. koreanum on cardiovascular disease. Therefore we investigated the vasorelaxant effects of O. koreanum and compared the vasorelaxant effects of four species of Gangwhal. Methods : The vasorelaxant effects of the ethanol extracts of Ostericum koreanum (NK and BK), Notopterygium incisium (NI), and N. forbesii (NF) on phenyleprine ($1{mu}M$) or KCl (60mM) pre-contracted rat thoracic aorta rings were compared. The vasorelaxant effects of the water extract and ethanol extract of NK on phenyleprine ($1{mu}M$) or KCl (60mM) pre-contracted rat thoracic aorta rings were compared. And the vasorelaxant effects of chloroform, ethylacetate and water fraction of ethanol extract of NK on phenyleprine ($1{mu}M$) or KCl (60mM) pre-contracted rat thoracic aorta rings were compared. Results : Ethanol extracts of NK, BK, NI and NF relaxed rat thoracic aorta rings with a concentration-dependent manner, and NK showed the greatest vasorelaxant effect. And ethanol extract of NK was much more effective than water extract of NK. Finally, chloroform, ethylacetate and water fraction of ethanol extract of NK also relaxed rat thoracic aorta rings, and chloroform fraction showed the greatest vasorelaxant effect.