Disclosed herein is a battery module assembly configured to have a structure in which a plurality of rectangular battery modules, each of which has two or more battery cells or unit modules connected in series and/or in parallel to each other, are stacked by two or more in a width direction (a longitudinal direction) thereof and in a height direction (a transverse direction) thereof so that the rectangular battery modules generally constitute a hexahedral structure (a hexahedral stack), outer edges of the hexahedral stack are fixed by a frame member, and coupling parts for mounting, through which the battery module assembly is mounted to an external device, are provided at one side of the frame member.