Disclosed herein is a battery pack having a plurality of battery cells or unit modules (‘unit cells’) electrically connected to each other, wherein the unit cells are arranged in a width direction (traverse direction) of the battery pack in an upright fashion in a state in which the unit cells are spaced apart from each other to form coolant flow channels, thereby constituting a battery module layer, and two or more battery module layers are mounted in a pack case such that the battery module layers are stacked in a height direction (vertical direction) of the battery pack, and wherein the battery pack includes a coolant introduction part, through which a coolant is introduced into the two or more battery module layers to simultaneously cool some or all of the unit cells in the two or more battery module layers, and a coolant discharge part, through which the coolant, having passed the unit cells, is discharged.