Human ether-á-go-go-related gene (hERG) K⁺ channel current (I _hERG ) is inhibited by various compounds and genetic mutations, potentially resulting in cardiac arrhythmia. Here, we investigated effects of caffeic acid phenethyl ester (CAPE) and curcumin, two natural anti-inflammatory polyphenols, on I _hERG in HEK-293 cells overexpressed with hERG. CAPE dose-dependently decreased repolarization tail current of hERG (I _hERG,tail; IC₅₀, 10.6 ± 0.5 μM). CAPE also shifted half-activation voltage (V _1/2) to the left (from −17.5 to −26.5 mV) and accelerated activation and inactivation kinetics. The CAPE inhibition of I _hERG,tail was not attenuated in the pore-blocker site mutants of hERG (Y652A and F656A). A point mutation of Cys723 (C723S) mimicked the effects of CAPE and caused a left shift of V _1/2 and acceleration of I _hERG,tail deactivation. However, I _hERG,tail inhibition by CAPE was still observed in C723S. Taken together, CAPE inhibits hERG channel by class 3 mechanism, i.e., modification of gating, not by blocking the pore. Curcumin induced changes of I _hERG similar to those of CAPE, while additional interaction with pore-blocking sites was suggested from attenuated I _hERG,tail inhibition in Y652A and F656A. Interestingly, I _hERG induced by human action potential voltage clamp was increased by CAPE while decreased by curcumin. Mathematical simulation of action potential derived from the experimental results of CAPE and curcumin supports that CAPE, but not curcumin, would induce shortening of AP duration by facilitation of I _hERG . The above results revealed intriguing roles of Cys723 in hERG kinetics and suggested that conventional drug screening by using step pulse protocol for I _hERG,tail would overlook the hERG kinetic modulations that could compensate the decrease of I _hERG,tail.