A cooling duct of a high voltage battery system of a vehicle is provided. The cooling duct includes a duct body that forms an inlet open towards the interior of the vehicle and an outlet that is formed at a side of the duct body opposite to the inlet to supply air introduced into the cooling duct through the inlet to a high voltage battery. A blocking wall is formed between the inlet and the outlet to restrict rectilinear communication between the inlet and the outlet while allowing air flow between the inlet and the outlet.