Disclosed is a battery pack including battery modules arranged in two or more rows, each of the battery modules including a plurality of battery cells or unit modules, each of which has two or more battery cells mounted therein, stacked in an upright or upside-down fashion, wherein the battery modules are individually mounted in pack cases, the pack cases are provided at upper parts and lower parts thereof with coolant inlet ports and coolant outlet ports such that a coolant to cool the battery cells flows to one side to the other side of the battery modules in a direction perpendicular to the stacked direction of the battery cells or the unit modules, the pack cases are further provided with flow spaces (‘coolant introduction parts’).