A method of fabricating a semiconductor device includes partially removing an active region and an isolation region to form a gate buried trench, forming a gate insulating layer on an inner wall of the gate buried trench, forming a gate conductive pattern on the gate insulating layer to fill the gate buried trench, and a height of an uppermost surface of the gate conductive pattern is lower than a height of an uppermost surface of the substrate. The method also includes forming an interlayer insulating layer on the substrate and on the gate conductive pattern, the interlayer insulating layer includes an upper insulating region and a lower insulating region, the lower insulating region fills the gate buried trench, the upper insulating region is formed over the substrate, and forming a bit contact plug connected to the active region through the interlayer.