Cinnamic acid (CA) and its derivatives have a broad therapeutic spectrum that includes antimicrobial, antifungal, and antitumoral activities. However, the vasodilative effect of CA has not been demonstrated. The present study characterizes the vasodilative activity and the mechanism of CA in rat thoracic aorta. The vasomotion of aortic strips following CA treatment was measured in an organ bath system. In addition, vascular strips and human umbilical vein endothelial cells (HUVECs) were used in organ bath, Western blot, nitrite, and cyclic guanosine monophosphate (cGMP) measurements. CA relaxed phenylephrine-precontracted aortic strips in an endothelium-dependent manner. Pretreatment of the endothelium-intact aortic strips with NG-nitro-l-arginine methyl ester (10−4 M), 1 H-[1,2,4]-oxadiazolole-[4,3-a] quinoxalin-10-one, (10−6 M) and methylene blue (10−5 M) inhibited CA-induced vasorelaxation. CA also increased the phosphorylation of endothelial nitric oxide synthase and nitric oxide generation in a concentration-dependent manner in HUVECs. In addition, cGMP generation and cGMP-dependent protein kinase G (PKG) expression in aortic strips were increased by CA treatment. Furthermore, CA-induced vasorelaxation was inhibited by the PKG inhibitor KT5823 (0.3 μM) and the Ca2+-activated K+ channel inhibitor tetraethylammonium (10−3 M). These findings suggest that CA exerts an endothelium-dependent vasodilation effect via the nitric oxide–cGMP–PKG-mediated pathway in rat thoracic aorta. Copyright © 2012 John Wiley & Sons, Ltd.