Disclosed is a battery module assembly for a vehicle's battery pack, which has four battery modules, among which two battery modules are arranged in parallel and two battery modules are stacked and provided on the two battery modules arranged in parallel, wherein cylindrical secondary battery cells respectively included in the battery modules are electrically connected in parallel by means of plates respectively disposed at tops of the two battery modules arranged in parallel, wherein the two battery modules arranged in parallel are electrically connected in parallel by means of a lower metal plate disposed on the two battery modules arranged in parallel, wherein cylindrical secondary battery cells respectively included in the battery modules are electrically connected in parallel by means of metal plates respectively disposed at tops and bottoms of the two battery modules stacked on the two battery modules arranged in parallel, and wherein the four battery modules are electrically connected in series. Therefore, it is possible to provide a stable and economic battery module assembly including a plurality of secondary battery cells.