A method of controlling an active air flap, includes detecting a location of a vehicle from a GPS signal received through a GPS receiver, and acquiring a road surface gradient of a road on which the vehicle travels from location information of the vehicle and map information stored in a data storage, by a controller, determining an opening degree setting value corresponding to the road surface gradient of the road, by the controller, outputting a control signal for controlling an opening degree of an air flap according to a setting value, by the controller, and controlling the opening degree of the air flap by operating an actuator according to the control signal.