Abstract: A water level detection apparatus for a secondary water tank of a radiator of a wheeled vehicle includes a water level detection element mounted in the secondary water tank, and a warning device connected to and controlled by the water level detection element to provide a warning effect when the water level detection element detects that the water level of the secondary water tank is smaller than the predetermined value. Thus, when the cooling water of the cooling system of the wheeled vehicle is located at the lower water level due to leakage or evaporation, the warning device is started to emit a warning signal to inform the driver of the abnormal condition, so that the driver can replenish the cooling water and check the circulating system of the water tank.