Provided a proton exchange membrane fuel cell (PEMFC) stack with at least two cell units. Each cell unit includes: a catalyzed membrane formed by catalyzing both surfaces of an electrolyte layer; a fuel flow field portion and air flow field portion formed at opposite surfaces of the catalyzed membrane, at least one of the fuel and air flow field portions comprising a parallel flow field which induces gas flow in a direction parallel to the surface of the catalyzed membrane and an orthogonal flow field which induces gas flow in a direction orthogonal to the surface of the catalyzed membrane; a bipolar plate in contact with each outer surface of the fuel flow region portion and the air flow region portions. The volume and weight of the entire fuel cell stack can be flexibly reduced by adjusting the number of cell units. In addition, a defective cell unit can be easily replaced without performance degradation in the cell stack. The use of bipolar plates as electrodes promotes gas flow. The PEMFC stack can effectively dissipate the reaction heat with a cooling member and can increase the power density per unit weight and volume. In particular, a pressure drop can be prevented with a serpentine air flow channel formed in the air flow field region, thereby increasing fuel conversion efficiency.