A method and apparatus to authenticate a registered user are described. The method and apparatus include a processor configured to identify a first electrocardiogram (ECG) signal measured from the user, and determine a similarity between the first ECG signal and a second ECG signal based on the identified first ECG signal and the second ECG signal included in a reference ECG signal set. The processor is also configured to determine an authentication threshold corresponding to the reference ECG signal set, and determine whether to authenticate the first ECG signal measured from the user by comparing the determined similarity and the authentication threshold.