A rack mountable server apparatus is disposed on a plane of a rack. The rack mountable server apparatus includes a chassis and a plurality of server units. The chassis is disposed in the rack, and the appearance of the chassis is a rectangular cuboid. When the chassis is installed in the rack, a first surface constructed by a length and a width of the chassis is parallel to the plane. The server units are disposed in the chassis. Each of server units has a circuit board, and the circuit boards are arranged parallel to a second surface which is constructed by the length and the height of the chassis or the width and the height of the chassis, and are perpendicular to the first surface.