A method of recognizing a trailer of a tractor vehicle by an electronic control unit (ECU) includes determining whether a parking switch is in an on state and a transmission gear state is a parking state. Whether a stop lamp switch is in an on state and a state of a current flowing in the stop lamp of the trailer is in an on state are determined. A load current quantity of a load lamp is recognized, and whether the stop lamp is in an open state is determined. A mode to a trailer non-connection mode when the stop lamp is in an off state is determined.