The current study aimed to investigate the role of K + channels subtypes in relaxant effects of catechin on rat's aortic rings. The isometric tension of the thoracic aorta was measured using ADI PowerLab Data Acquisition System. Catechin at concentrations (10-5 to 5x10-1 M) produced more potent relaxant effects in phenylephrine (10-6 M) precontracted aortic smooth muscle with Log IC 50 's of-1.159 mg/ml as compared to potassium chloride (60 mM) with Log IC 50 's of-6.373 mg/ml. The catechin induced relaxation in aortic rings precontracted with phenylephrine was 17.464 ± 0.068 %, where as for aortic rings precontracted with potassium was only 5.574 ± 0.131%. In aortic rings preincubated with glibenclamide (10-5 M) and tetraethylammonium (1mM), catechin relaxant effect was enhanced with Log IC 50 s of-3.119 and-3.001 mg/ml, respectively. On the other hand, in aortic rings precontracted with PE and preincubated with BaCl 2 and 4-AP, catechin induced statistically non-significant relaxant effects as compared with aortic rings of the control aortic ring which was precontracted with PE. From the results of the current study, it can be concluded that catechin produced more potent vasorelaxant effect on aortic rings preincubated with phenylephrine that KCl. This vasorelaxant effect of catechin is produced via the activation of both K ATP and Kca, but not Kir and Kv channels subtypes.