A fuel cell stack having an improved sealing structure of a cooling plate in which the cooling plates therein each have a coolant flow channel through which a coolant flows to remove heat from the fuel cell stack and a groove that surrounds the coolant flow channel, and a sealing member disposed in the groove to prevent coolant leakage. The sealing member has a compression rate of 18 to 30%, as the compression rate being ((a thickness of the sealing member−a height of the groove)/the thickness of the sealing member)×100. Such a fuel cell stack may maintain operation for a long time without the need of supplementing the coolant.