A portable electronic device to be used as a keyless device of a vehicle includes a controlling circuit which includes a physiological feature detection module and a processing circuit. The physiological feature detection module is used for detecting a physiological feature of a person. The processing circuit is coupled to the physiological feature detection module and used for determining whether the person is an authorized user of the vehicle according to the detected physiological feature of the person, and for opening a car door of the vehicle or starting the vehicle when the person is identified by the processing circuit as the authorized user.