The present invention features a battery pack, that is provided to supply electricity to a variety of electric parts, such as a drive motor, installed in an environment friendly electric vehicle such as a fuel cell vehicle or a hybrid electric vehicle. Preferably, the battery pack is configured to stack cell assemblies. Each cell assembly includes a cell case. The cell case includes windows in opposite surfaces thereof such that unit cells are exposed, and recesses in opposite long lateral faces thereof such that the windows communicate with an outside. The windows and the recesses define cooling passages between the stacked cell assemblies.