Despite the increasing attention on the therapeutic potential of Curcuma longa (turmeric), the biological activities of curcuminoids other than curcumin are not well understood. Here, we investigated antivasoconstrictive activities of C. longa extract and its ingredients using freshly isolated rat aortic rings. C. longa extract significantly suppressed agonist-stimulated vasoconstriction, and cyclocurcumin was found to be the most potent (IC50 against phenylephrine-induced vasoconstriction: 14.9 ± 1.0 μM) among the 10 tested ingredients including four curcuminoids. Cyclocurcumin significantly inhibited contraction of vascular smooth muscle, which was mediated by the suppression of myosin-light-chain phosphorylation and calcium influx via the L-type calcium channel. The inhibitory effect of cyclocurcumin was observed to be reversible and without cytotoxicity. Taken together, we demonstrated that cyclocurcumin, a bioactive ingredient in C. longa, may have a therapeutic potential as a novel antivasoconstrictive natural product.