3,4,5-Trimethoxycinnamic acid (TMCA), methyl 3,4,5-trimethoxycinnamate (M-TMCA) and p-methoxycinnamic acid (PMCA) have been identified as the major bioactive components in the serum collected from rats treated with oral administration of Polygalae Radix (“YuanZhi,” the roots of Polygala tenuifolia WILLD.), a traditional Chinese medicine used to relieve insomnia, anxiety and heart palpitation. The present study was designed to investigate its direct electrophysiological effects on isolated ventricular myocytes from rabbits. Whole-cell configuration of the patch-clamp technique was used to measure action potential (AP) and membrane currents in single ventricular myocytes enzymatically isolated from adult rabbit hearts. Ca²⁺ transients were recorded in myocytes loaded with the Ca²⁺ indicator Fluo-4AM. Among three bioactive substances of Polygala metabolites, only M-TMCA (15–30 µM) significantly shortened action potential duration at 50% and 90% repolarization (APD₅₀ and APD₉₀) in cardiomyocytes in a concentration-dependent and a reversible manner. M-TMCA also inhibited L-type calcium current (I_Ca,L), but showed effect on neither transient outward potassium current (I_to) nor steady-state potassium current (I_K,SS). Furthermore, M-TMCA abolished isoprenaline plus BayK8644-induced early afterdepolarizations (EADs) and suppressed delayed afterdepolarizations (DADs) and triggered activities (TAs). This potential anti-arrhythmic effects were likely attributed by the inhibition of I_Ca,L and the suppression of intracellular Ca²⁺ transients, which consequently suppress the generation of transient inward current (I_ti). These findings suggest that M-TMCA may protect the heart from arrhythmias via its inhibitory effect on calcium channel.