A battery pack including battery cells or unit modules (unit cells), configured to have a structure including a battery module group including one or more battery modules each having the unit cells mounted in a pack case in a state in which the unit cells are uprightly arranged in a width direction of the pack such that the unit cells are spaced apart by a spacing distance for coolant flow, a coolant introduction part continuously defined in a space between a bottom of the pack case and the battery module group, a coolant discharge part defined between a top of the pack case and the battery module group, an electronic member located at one side of the module group, the electronic member being mounted in an inner space defined by the coolant discharge part, and a coolant flow channel defined between the coolant introduction part and the coolant discharge part.