A battery pack comprising: (a) a module assembly comprising two or more battery modules (401, 402, 403, 404, 405, 406), each of which comprises a chargeable and dischargeable battery cell, the battery modules being stacked to have a two layer structure comprising an upper layer and a lower layer while being in contact with each other in a lateral direction; characterized in that said battery pack further comprises: (b) a first upper layer connection member mounted at the upper layer module to surround a portion of opposite sides and a front upper end of outermost battery modules (401, 403) of the upper layer module assembly, and a second upper layer connection member mounted at the upper layer module assembly to surround a portion of the opposite sides and a rear upper end of the outermost battery modules (401, 403) of the upper layer module assembly; (c) a first lower layer connection member mounted at the lower layer module assembly to surround a portion of opposite sides and a front lower end of outermost battery modules (404, 406) of the lower layer module assembly, and a second lower layer connection member mounted at the lower layer module assembly to surround a portion of the opposite sides and a rear lower end of the outermost battery modules (404, 406) of the lower layer module; (d) a pair of side support members (810, 820) coupled to the upper layer connection members (510, 520) and the lower layer connection members (530, 540) to simultaneously surround sides of the upper and lower layer module assemblies; (e) insulation members (710, 720) mounted at interfaces between the sides of the upper and lower layer module assemblies and the side support members (810, 820); and (f) a first lower end support member coupled to the side support members to support the front lower end of the lower layer module assembly, and a second lower end support member coupled to the side support members to support the rear lower end of the lower layer module assembly. (a) a module assembly comprising two or more battery modules (401, 402, 403, 404, 405, 406), each of which comprises a chargeable and dischargeable battery cell, the battery modules being stacked to have a two layer structure comprising an upper layer and a lower layer while being in contact with each other in a lateral direction; characterized in that said battery pack further comprises: (b) a first upper layer connection member mounted at the upper layer module to surround a portion of opposite sides and a front upper end of outermost battery modules (401, 403) of the upper layer module assembly, and a second upper layer connection member mounted at the upper layer module assembly to surround a portion of the opposite sides and a rear upper end of the outermost battery modules (401, 403) of the upper layer module assembly; (c) a first lower layer connection member mounted at the lower layer module assembly to surround a portion of opposite sides and a front lower end of outermost battery modules (404, 406) of the lower layer module assembly, and a second lower layer connection member mounted at the lower layer module assembly to surround a portion of the opposite sides and a rear lower end of the outermost battery modules (404, 406) of the lower layer module; (d) a pair of side support members (810, 820) coupled to the upper layer connection members (510, 520) and the lower layer connection members (530, 540) to simultaneously surround sides of the upper and lower layer module assemblies; (e) insulation members (710, 720) mounted at interfaces between the sides of the upper and lower layer module assemblies and the side support members (810, 820); and (f) a first lower end support member coupled to the side support members to support the front lower end of the lower layer module assembly, and a second lower end support member coupled to the side support members to support the rear lower end of the lower layer module assembly.