A method and charging system for effectively performing measurement of a pulse width modulation (PWM) signal for selection of a charging mode between an electric vehicle and a charger are disclosed. The method for determining a charging mode of an electric vehicle includes measuring a duty ratio of a control signal applied to a communication line of a charger connector by a first controller for controlling slow charging and a second controller for controlling quick charging, transmitting measurement results obtained from each of the first controller and the second controller to a third controller in charge of a battery, determining a final duty ratio using the measurement results by the third controller, transmitting the determined final duty ratio to the first controller and the second controller, and starting charging in a mode corresponding to the final duty ratio.