Disclosed herein is a middle- or large-sized battery pack including a module assembly constructed in a structure in which a plurality of battery modules, each of which includes a plurality of battery cells or unit modules mounted in a module case while the battery cells or unit modules are connected in series to each other, are arranged such that the battery modules are disposed in contact with each other in the lateral direction, and a coolant flow channel is vertically formed, a plurality of support members for supporting opposite sides and the bottom of the module assembly and maintaining the arrangement state of the module assembly, and a pack housing for surrounding the module assembly and the support members, wherein the battery pack is constructed in a cooling structure in which a coolant is introduced through one side upper end (or lower end) of the module assembly along a hermetically sealed space defined by the support members, flows vertically through the module assembly, and is discharged through the other side lower end (or upper end) of the battery assembly.