Disclosed herein is a battery pack including a plurality of battery modules, each having a battery cell or a unit module (unit cell) that can be charged and discharged, mounted in a pack case, wherein two or more unit cells constitute one battery module, two or more battery modules are arranged in a length direction of the battery pack to constitute one battery module group, two battery module groups are arranged in a width direction of the battery pack in a state in which the battery module groups are spaced apart from each other such that a coolant discharge part is defined between the battery module groups, a coolant inlet port is independently formed at a region of the pack case corresponding to each of the battery modules located at a position opposite to the coolant discharge part, and a coolant outlet port is formed at a front or a rear of the pack case in the length direction of the battery pack such that coolant introduced through the respective coolant inlet ports cools the unit cells of the respective battery modules while passing through the respective battery modules and is then discharged out of the pack case.