A furocoumarin derivative, oxypeucedanin, was purified from Angelica dahurica, and its effects on the human Kv1.5 (hKv1.5) channel and on the cardiac action potential duration (APD), were examined using the patch-clamp technique and the conventional microelectrode technique. Oxypeucedanin inhibited the hKv1.5 current in a concentration-dependent manner, with an IC(50) value of 76 nM, while it had no effect on human eag-related gene (HERG) current. Oxypeucedanin induced an initial fast decline of hKv1.5 current during depolarizations. The inhibition of hKv1.5 channel by oxypeucedanin was voltage-dependent, especially at depolarizing pulses between -40 and 0 mV which corresponds to the voltage range of the channel's opening. Oxypeucedanin also slowed the deactivation time course, resulting in a tail crossover phenomenon. Additionally, oxypeucedanin prolonged the APD of rat atrial and ventricular muscles in a dose-dependent manner. These results suggest that oxypeucedanin is a kind of open-channel blocker of the hKv1.5 channel and it prolongs the APD; therefore, it is an excellent candidate as an antiarrhythmic drug for atrial fibrillation.