We investigated the vasorelaxant effect of cilostazol and related signaling pathways in phenylephrine (Phe)-induced pre-contracted aortic rings. Cilostazol induced vasorelaxation in a concentration-dependent manner when aortic rings were pre-contracted with Phe. Application of the voltage-dependent K+ (Kv) channel inhibitor 4-AP, the ATP-sensitive K+ (KATP) channel inhibitor glibenclamide, and the inwardly rectifying K+ (Kir) channel inhibitor Ba2+ did not alter the vasorelaxant effect of cilostazol; however, pre- and post-treatment with the big-conductance Ca2+-activated K+ (BKCa) channel inhibitor paxilline inhibited the vasorelaxant effect of cilostazol. This vasorelaxant effect of cilostazol was reduced in the presence of an adenylyl cyclase or a protein kinase A (PKA) inhibitor, but not a protein kinase G inhibitor. Inside-out single channel recordings revealed that cilostazol induced the activation of BKCa channel activity. The vasorelaxant effect of cilostazol was not affected by removal of the endothelium. In addition, application of a nitric oxide synthase inhibitor and a small-conductance Ca2+-activated K+ (SKCa) channel inhibitor did not affect cilostazol-induced vasorelaxation. We conclude that cilostazol induced vasorelaxation of the aorta through activation of BKCa channel via a PKA-dependent signaling mechanism independent of endothelium.