NOVELTY - A passive cooling system for a fuel cell stack (120), comprises cooling plates (130), each installed between unit cells (110) and having an inlet hole through which a primary coolant enters, an outlet hole through which the coolant leaves, and flow channels between holes; and a heat exchanger (150) installed on a primary coolant flow line (140) connected from outlet holes to inlet holes of plates to change a vapor state primary coolant to a liquid state primary coolant by cooling the coolant; the flow of the coolant is achieved by natural convection caused by vaporization of the coolant.    USE - For fuel cell stack, such as polymer electrolyte membrane fuel cell (PEMFC) useful for automotive, portable, residential and small commercial applications.    ADVANTAGE - The cooling system does not use a pump for circulating a primary coolant since the primary coolant is circulated by natural convection. Accordingly, the power required to operate the pump is eliminated, and the size of the fuel system can be reduced.    DETAILED DESCRIPTION - A passive cooling system for a fuel cell stack (120), comprises several cooling plates (130), each installed amongst unit cells (110), and having flow channels, an inlet hole through which a primary coolant enters and an outlet hole through which the primary coolant that has passed through the flow channels leaves; and a heat exchanger (150) installed on a primary coolant flow line (140) connected from the outlet holes to the inlet holes of the cooling plates to change a vapor state primary coolant to a liquid state primary coolant by cooling the primary coolant. A path through which the primary coolant passes is a closed circuit, and the flow of the primary coolant is achieved by natural convection caused by vaporization of the primary coolant.    DESCRIPTION OF DRAWING(S) - The figure shows a perspective view of a passive cooling system for a fuel cell stack.    Unit cells (110)    Fuel cell stack (120)    Cooling plates (130)    Primary coolant flow line (140)    Heat exchanger (150)    Coil shape cooling pipe. (256)