It is important for fuel cell to become flexible in order to accommodate the situations of application areas for reducing costs and expanding coverage. Scale ups in series and parallel to 1-kW class proton exchange membrane (PEM) fuel cell stack are characterized experimentally with oxidant as air and oxygen. Through a serial scaled-up 4-cell stack with an active area of 25 cm(2) and a parallel scaled-up single cell with an active area of 100 cm(2), respectively, the 1-kW class 20-cell stack with an active area of 150 cm(2) per cell is scaled up from the basic unit cell with maintaining their performances. The polarization and power curves of the 1-kW class PEM fuel cell stack with the reactants H-2/air and H-2/O-2 are evaluated. Sufficient power of the PEM fuel cell can operate at flexible nominal current of 40-80 A and nominal voltage of 12-15 V for providing durability and balance between cells. The 1-kW class PEM fuel cell stack is characterized through the comparison of cell voltage differences utilizingH(2)/air andH(2)/O-2. Both cell voltages with H-2/air and H-2/O-2 follow well the one-twentieth of the responding stack voltages with the small difference.