Disclosed herein is a battery module assembly including a plurality of battery modules, each including a plurality of battery cells or unit modules mounted in a module case in a state in which the battery cells or the unit modules are connected in series to each other, wherein the battery modules are arranged adjacent to each other in the lateral direction in a state in which the battery modules are electrically connected to each other, and a cooling member including a coolant conduit to allow a liquid coolant to flow therealong is mounted at the outside of each of the battery modules.