A frame jamb marker includes a sliding panel for sliding on the side surface of the frame and a guiding panel perpendicularly extended from the sliding panel for sliding on the marking surface of the frame, wherein the guiding panel has a top stepping edge and a bottom stepping edge to define a top marking point and a bottom marking point at stepping corners at the top and bottom stepping edges respectively. The user is able to point a marker tool on the marking surface at one of the top and bottom marking points. When the sliding panel is slid on the side surface to drive the guiding panel sliding on the marking surface, the marker tool is guided to slidably mark on the marking surface to make a straight marking line thereon so as to precisely form the marking line parallel to the common edge of the frame.