Disclosed herein is a middle- or large-sized battery pack having two or more hexahedral battery modules, each including a plurality of chargeable and dischargeable plate-shaped battery cells or unit modules (unit cells), mounted in a pack case, wherein the battery pack is used as a power source for driving a vehicle, has a battery capacity of 5 KWh or more, and removes heat generated during the charge and discharge of the unit cells using an air cooling method, the unit cells of each of the battery modules are vertically stacked such that the number of the unit cells corresponds to a height of a battery pack installation space defined in the vehicle while neighboring unit cells are spaced apart from each other such that a coolant channel is defined between the respective unit cells, the pack case is formed in a shape corresponding to the battery pack installation space in the vehicle, the hexahedral battery modules being arranged in an inner space of the pack case, and a coolant for removing heat from the unit cells is introduced through at least one side of the pack case, passes through the battery modules horizontally, and is discharged through the other side of the pack case.