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 deceased repolarization tail current of hERG (IhERG,tail; IC50, 10.6 ± 0.5 μM). CAPE also shifted half-activation voltage (V1/2 ) to the left (from -17.5 to -26.5 mV), and accelerated activation and inactivation kinetics. The CAPE inhibition of IhERG,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; left shift of V1/2 and acceleration of IhERG,tail deactivation. However, IhERG,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 IhERG similar to those of CAPE while additional interaction with pore blocking sites was suggested from attenuated IhERG,tail inhibition in Y652A and F656A. Interestingly, IhERG induced by human action potential voltage clamp was increased by CAPE while decreased by curcumin. Above results revealed intriguing roles of Cys723 in hERG kinetics, and suggested that conventional drug screening by using step pulse protocol for IhERG,tail would overlook the hERG kinetics modulations that could compensate the apparent decrease of IhERG,tail.