Previously, we found that a furocoumarin derivative, psoralen (7H-furo[3,2-g][1 ]benzopyran-7-one), blocked a human Kv1.5 potassium channel (hKv1.5) and has a potential antiarrhythmic effect. In the present study, to develop more potent hKv1.5 blockers or antiarrhythmic drugs, we synthesized ten psoralen derivatives and examined their blocking effects or hKv1.5 stably expressed in Ltk cells. Among the newly synthesized psoralen derivatives, three derivatives (Compounds 5,9 and 10) showed the open channel-blocking effect. Compound 9 among them was the most potent In blocking hKv1.5. We found that compound 9, one of the psoralen derivatives, inhibited the hKv1.5 current in a concentration-, use- and voltage-dependent manner with an IC$_{50}$ value of 27.4 ${pm}$ 5.1 nM at +60 mV. Compound 9 accelerated the inactivation kinetics of the hKv1.5 channel, slowed the deactivation kinetics of hKv1.5 current resulting in a tail crossover phenomenon. Compound 9 inhibited hKv1.5 current in a use-dependent manner. These results indicate that compound 9, one of psoralen derivatives, acts on hKv1.5 channel as an open charnel blocker and is much more potent than psoralen in blocking hKv1.5 channel. If further studios were done, compound 9 might be an ideal antiarrhythmic drug for atrial fibrillation.