A vehicle battery pack assembly includes a battery module including a plurality of battery cells stacked and provided in a transverse direction, and an endplate provided at an outermost side of the battery module in a width direction of the battery module and formed with a mounting portion protruding outward therefrom; and a battery casing provided with a casing member extending along a longitudinal direction of the battery module, the casing member, of which a surface facing toward the battery module is in surface-contact with the endplate to support a surface pressure of the battery module, wherein the casing member includes a fastening groove formed at a point corresponding to the mounting portion, wherein the mounting portion is inserted in the fastening groove and fastened to the casing member.