An overlay mark is described, including a portion of a lower layer having two x-directional and two y-directional bar-like patterns therein, and two x-directional and two y-directional photoresist bars defined by the lithography process for defining an upper layer and surrounded by the bar-like patterns. At least one of the patterning process for defining the lower layer and the above lithography process includes two exposure steps respectively for defining a first device area and a second device area. When the patterning process includes two exposure steps, one x-directional and one y-directional bar-like patterns are defined simultaneously and the other x-directional and the other y-directional bar-like patterns are defined simultaneously. When the lithography process includes two exposure steps, one x-directional and one y-directional photoresist bars are defined simultaneously and the other x-directional and the other y-directional photoresist bars are defined simultaneously.