In this study, ECG signals of normal sinus rhythm (NSR), ventricular tachycardia (VT) and ventricular fibrillation (VF) were taken out to extract the RR interval series. Approximate entropy (ApEn) values of the RR interval series were calculated and compared among the three groups by ANOVA methods. It was found that the values of ApEn were increased significantly from NSR to VT and then to VF. Approximate entropy may be regarded as an index to discriminate the ECG signals of different state.