Disclosed herein is a battery pack including a battery module assembly configured to have a structure in which two or more battery modules, each of which includes battery cells arranged in an upright state such that electrode terminals of the battery cells are directed upward, are arranged such that the battery modules are adjacent to each other, and outward protrusions are formed at a lower end of one side or lower ends of opposite sides of each of the battery modules, a base plate, on a top of which the battery module assembly is loaded, and a hold down bracket having one side region covering the outward protrusions of the battery modules and the other side region coupled to the base plate so as to correctly fix the respective battery modules of the battery module assembly to the base plate.