In a cooling system for cooling a battery pack mounted in a vehicle, the battery pack is mounted at an inner lower end of the vehicle constructed in a corrugated structure including alternating valleys and peaks. Battery modules of the battery pack are arranged such that unit cells of each battery module are mounted vertically on the corrugated lower end of the vehicle. The cooling system includes a coolant inlet port and a coolant outlet port formed at a lower-end member of a housing surrounding an outer surface of the battery pack. The coolant inlet port and the coolant outlet port are formed using a structure of the vehicle.