In actual size of polymer electrolyte fuel cell stack with heat management of cooling water, relative humidity distribution was calculated under various kinds of operating conditions and various shapes of gas channel with numerical analysis. And the optimal separator shape and the optimal flow pattern of gas and cooling water that make the relative humidity higher and more uniform and that lead to the improvement of cell durability were examined under each operating condition. As a result, the effects of humidify temperature, the temperature of cooling water at an outlet and the average current density on humidity distribution which was affected by vapor concentration and gas temperature were examined and it was found that the optimal combination of flow pattern of gas and cooling water was the same under each operating condition. As regards the operating condition in this paper, the relative humidity is the highest and the most uniform in the following cases: gas flow pattern is counter, the cooling water is synchronized with cathode gas flow and the ordinary serpentine separator with 1.0 mm depth channels is used in cathode and anode sides. However, it is found that it is possible to occur the flooding in such cases. (c) 2006 Elsevier B.V. All rights reserved.