It had been known for a while that Crataegi Fructus(CF; Crataegus pinnatifida Bunge) had only a digestive effect. Recently, it has been demonstrated that CF also has an anti-hypertensive effect. However, its mechanism of relaxant effect has not been investigated yet. This study was examined to investigate the mechanism of vascular relaxation effect of CF in isolated rat thoracic aorta. CF revealed significant relaxation to phenylephrine(PE)-induced arterial contraction but much less to KCI-induced one. When CF was pretreated, it inhibited PE-induced contraction non-competitively. Methylene blue(10^-6M) completely blocked the relaxant effect of CF whereas L-NAME(10^-5M) did almost completely. However, atropine(10^-6M) did not have any influence on vascular relaxation effect of CF. Regarding cNOS activity, CF significantly increased its activity from rat whole brain homogenate in a dose dependent manner which was inhibited by L-NAME(10^-5M). On the other hand, CF did not affect on expression of TNF-α mRNA in RAW 264.7 cells, suggesting that CF is not related to iNOS activity. These results indicate that CF would be effective in relaxing vascular contraction through release of endothelial nitric oxide.