An assembly including a motor housing assembly and a motor assembly mounted in the motor housing assembly, the motor housing assembly comprising: an aluminum housing coupled to an outer circumferential surface of the motor assembly and configured to form a first space between the aluminum housing and the motor assembly, the first space including an oil flow channel; a plastic housing coupled to an outer circumferential surface of the aluminum housing and configured to form a second space between the plastic housing and the aluminum housing, the second space including a water flow channel; a rear bracket coupled to the aluminum housing at one axial end of the aluminum housing; and a rib configured to radially extend from an opposite axial end of the aluminum housing, wherein the aluminum housing comprises a plurality of partitions protruding from the outer circumferential surface in a radially outward direction, each of the partitions extending to a predetermined length in an axial direction of the motor assembly and configured to form the water flow channel, each of the partitions including a second joining surface to be in contact with an inner circumferential surface of the plastic housing; wherein a first partition of a pair of adjacent partitions selected from the plurality of partitions extends axially from one axial end of the aluminum housing, and a second partition of the pair of adjacent partitions extends axially from an opposite axial end of the aluminum housing; and wherein the plastic housing is configured to be coupled to the aluminum housing through press-fitting; wherein the motor housing assembly further comprises: an outer screw hole formed in the rear bracket; and an inner screw hole formed in an area corresponding to the outer screw hole in the aluminum housing, wherein the inner screw hole is formed in the partition, and wherein escaping of the plastic housing in an axial direction is prevented by the rear bracket contacting one axial end of the plastic housing and the rib contacting an opposite axial end of the plastic housing. an aluminum housing coupled to an outer circumferential surface of the motor assembly and configured to form a first space between the aluminum housing and the motor assembly, the first space including an oil flow channel; a plastic housing coupled to an outer circumferential surface of the aluminum housing and configured to form a second space between the plastic housing and the aluminum housing, the second space including a water flow channel; a rear bracket coupled to the aluminum housing at one axial end of the aluminum housing; and a rib configured to radially extend from an opposite axial end of the aluminum housing, wherein the aluminum housing comprises a plurality of partitions protruding from the outer circumferential surface in a radially outward direction, each of the partitions extending to a predetermined length in an axial direction of the motor assembly and configured to form the water flow channel, each of the partitions including a second joining surface to be in contact with an inner circumferential surface of the plastic housing; wherein a first partition of a pair of adjacent partitions selected from the plurality of partitions extends axially from one axial end of the aluminum housing, and a second partition of the pair of adjacent partitions extends axially from an opposite axial end of the aluminum housing; and wherein the plastic housing is configured to be coupled to the aluminum housing through press-fitting; wherein the motor housing assembly further comprises: an outer screw hole formed in the rear bracket; and an inner screw hole formed in an area corresponding to the outer screw hole in the aluminum housing, wherein the inner screw hole is formed in the partition, and wherein escaping of the plastic housing in an axial direction is prevented by the rear bracket contacting one axial end of the plastic housing and the rib contacting an opposite axial end of the plastic housing. an outer screw hole formed in the rear bracket; and an inner screw hole formed in an area corresponding to the outer screw hole in the aluminum housing, wherein the inner screw hole is formed in the partition, and wherein escaping of the plastic housing in an axial direction is prevented by the rear bracket contacting one axial end of the plastic housing and the rib contacting an opposite axial end of the plastic housing.