Abstract: A control button assembly mounted in the top side of a mechanical pen and linked to the mechanical mechanism of the mechanical pen is disclosed to include a base member coupled to the mechanical mechanism of the mechanical pen, a cap capped on the base member for pressing by the user to drive the mechanical mechanism to move a writing tip in and out of the bottom side of the mechanical pen, a tip member threaded onto the base member inside the cap and having two longitudinal side rails respectively coupled to respective longitudinal sliding grooves inside the cap and usable as a touch pen and movable in and out of the cap upon rotary motion of the cap.