An improved pivotal hinge mechanism used for portable computer equipped with a stopper device which when portable computer and a docking station are joined together shall be actuated by the docking station to prevent the LCD of the portable computer from rotating in left and right direction, so that the collision happened between the docking station and the LCD of the portable computer due to rotation of the LCD relative to the computer mainframe shall be avoided; when the portable computer is disconnected from the docking station said stopper device shall immediately return to the normal position so that the LCD of the portable computer can rotate again in an angle of 0–180 degrees in left and right direction with respect to the computer mainframe.