A semiconductor memory device has an asymmetric buried gate structure with a stepped top surface and a method for fabricating the same. The method for fabricating the semiconductor memory device includes: etching a predetermined region of a semiconductor substrate to form an isolation layer defining an active region; forming a recess within the active region; forming a metal layer filling the recess; asymmetrically etching the metal layer to form an asymmetric gate having a stepped top surface at a predetermined portion of the recess; and forming a capping oxide layer filling a remaining portion of the recess where the asymmetric gate is not formed, thereby obtaining an asymmetric buried gate including the asymmetric gate and the capping oxide layer.