Disclosed herein is a battery pack including (a) a module assembly including two or more battery modules, each of which includes a chargeable and dischargeable battery cell, the battery modules being stacked to have a two layer structure including an upper layer and a lower layer while being in contact with each other in a lateral direction, (b) a first upper layer connection member and a second upper layer connection member mounted at the upper layer module assembly, (c) a first lower layer connection member and a second lower layer connection member mounted at the lower layer module assembly, (d) a pair of side support members, (e) insulation members mounted at interfaces between the sides of the upper and lower layer module assemblies and the side support members, and (f) a first lower end support member and a second lower end support member.