An unitized reversible cell stack (URC) which can work as both water electrolyzer and fuel cell is an interesting device in the wide use of hydrogen energy in the near future. We have manufactured a single cell, as practical electrode size, with electrode area of 250cm². From the results of a cell performance evaluation, suitable conditions for high performance operation of the URC and the switching criteria of both modes were identified. Moreover, based on the results of single cells, we manufactured the 1kW-class URC stack. The round-trip efficiencies of both water electrolyzer and fuel cell modes of the URC stack were examined. From the obtained results, it was confirmed that the energy conversion efficiency of the URC developed here were comparable with that of the dedicated systems. In addition, it was demonstrated that the switching operation mode of the URC between water electrolyzer and fuel cell was performed successfully.