A battery module assembly comprising unit modules, each of which comprises unit cells mounted to a cartridge (120a, 120b) in a state of being electrically connected to each other via bus bars, the battery module assembly comprising: two or more sub-modules, a module case, in which the sub-modules are received and fixed, including an upper cover plate and a lower base plate, each of the sub-modules comprises two or more unit modules vertically stacked between the lower base plate and the upper cover plate 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 the module case has a coolant inlet port, through which a coolant is introduced into the module case, and a coolant 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, each of the unit cells is a plate-shaped battery cell or a battery cell assembly comprising two or more plate-shaped battery cells mounted in a cell cover in a state in which electrode terminals (21, 22) of the battery cells are exposed, the cell cover comprises a pair of sheathing members coupled to each other for covering outer surfaces of the battery cells excluding the electrode terminals (21, 22), and wherein the module case is provided at portions thereof perpendicular to the coolant outlet port with two or more coolant inlet ports arranged with respect to the coolant flow channel, wherein the coolant inlet ports are formed at opposite portions of the module case symmetrically with respect to the coolant flow channel, wherein one end of a surface of each of the bus bars, to which a corresponding one of the electrode terminals (110a, 110b) is coupled, is bent upward or downward, and the bent end is connected to a series connection member such that adjacent unit modules are connected in series to each other and wherein a fan or a discharge duct is mounted at the coolant outlet port of the battery module assembly. two or more sub-modules, a module case, in which the sub-modules are received and fixed, including an upper cover plate and a lower base plate, each of the sub-modules comprises two or more unit modules vertically stacked between the lower base plate and the upper cover plate 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 the module case has a coolant inlet port, through which a coolant is introduced into the module case, and a coolant 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, each of the unit cells is a plate-shaped battery cell or a battery cell assembly comprising two or more plate-shaped battery cells mounted in a cell cover in a state in which electrode terminals (21, 22) of the battery cells are exposed, the cell cover comprises a pair of sheathing members coupled to each other for covering outer surfaces of the battery cells excluding the electrode terminals (21, 22), and wherein the module case is provided at portions thereof perpendicular to the coolant outlet port with two or more coolant inlet ports arranged with respect to the coolant flow channel, wherein the coolant inlet ports are formed at opposite portions of the module case symmetrically with respect to the coolant flow channel, wherein one end of a surface of each of the bus bars, to which a corresponding one of the electrode terminals (110a, 110b) is coupled, is bent upward or downward, and the bent end is connected to a series connection member such that adjacent unit modules are connected in series to each other and wherein a fan or a discharge duct is mounted at the coolant outlet port of the battery module assembly.