Provided is a rail assembly of a refrigerator. The rail assembly includes a pair of extendable rails, a rack, pinions, and restricting units. The rails are disposed at left and right sides of an inner wall of a storage space. The rack is disposed under the rails. The rack includes teeth continuously formed in a length direction of the rails and a flat alignment part disposed at a front side of the rack. The pinions are respectively disposed at the rails and movable in mesh with the teeth of the rack. The restricting units are disposed at sides of the rails for restricting a movement of the rails when the rails are extended, so as to prevent the pinions from departing from the rack. When the rails are maximally extended, the pinions are placed at the alignment part.