Disclosed is an auto cruise downhill control (ADC) method for a vehicle. By the method, when the speed of the vehicle exceeds an auto cruise setting speed, the speed of the vehicle is automatically adjusted to be the auto cruise setting speed by using an engine control unit (ECU) and an electronic stability control (ESC) unit. In other words, when a current vehicle speed exceeds a predetermined value compared to a driver's setting speed for auto cruise when the vehicle is operating on a steep decline, automatic braking activation control is performed by the ESC unit to adjust the speed of the vehicle to be the auto cruise setting speed. Additionally, when a driver makes a steep downward adjustment of the setting speed, automatic braking activation control is performed by the ESC unit to decrease the speed of the vehicle to a downwardly adjusted level at an accelerated rate.