Disclosed herein is a battery module assembly including unit modules, each of which includes unit cells mounted to a cartridge in a state of being electrically connected to each other via bus bars, the battery module assembly including two or more sub-modules, each of which includes two or more unit modules vertically stacked from a ground to form a coolant flow channel at an interface therebetween, the unit modules being arranged in a lateral direction in a state of being spaced apart from each other to provide the coolant flow channel, and a module case, in which the sub-modules are received and fixed, the module case having a coolant inlet port, through which a coolant is introduced into the module case, and a cool outlet port, through which the coolant is discharged out of the module case, wherein the coolant flow channel is configured to have a structure in which a vertical sectional area of the coolant flow channel decreases toward the coolant outlet port.