An auto winding mechanism for a window blind includes two guide rollers pivotally mounted between an upper seat and a lower seat and connected by an elastic winding tape, and two pull cords wound around the pull cords. Each of the guide rollers has a gear and a bottom disk. The upper seat has a covering edge surrounding the gear of each of the guide rollers. The lower seat has a surface provided with two annular grooves to receive the bottom disk of each of the two guide rollers. Thus, each of the pull cords is protected by the bottom disk of each of the guide rollers so that when the pull cords swing or bump during movement, each of the pull cords will not be jammed between the lower seat and each of the guide rollers.