An interlocking component assembly of an expandable rack assembly is proposed, including a plurality of horizontally-deposed panels each formed with a first opening, a plurality of vertically-deposed panels each disposed between two horizontally-deposed panels, a plurality of middle joint components each disposed in vertically-deposed panels, and a plurality of interlocking components each adapted for interconnecting two middle joint components to interconnect and fasten the horizontally-deposed panel to the vertically-deposed panel, wherein the height of the vertically-deposed panel is greater than the length of the middle joint component, such that the vertically-deposed panel can be coupled to the interlocking component to securely engage the horizontally-deposed panel with the vertically-deposed panel, thereby enabling the rack assembly to be free from limitations of assembling locations or dimensional specifications while the structural rigidity and strength of the assembled rack unit is enhanced.