A semiconductor device includes a substrate, a stacked structure disposed on the substrate, and dummy memory string structures. The stacked structure includes alternately stacked insulating layers and conductive layers. The dummy memory string structures disposed in a staircase region of the semiconductor device penetrate the stacked structure along a first direction. The staircase region includes a body portion including a first region and a second region adjacent to the first region. In the first region, an amount of conductive layers corresponding to the dummy memory string structures is between 1 and 10; in the second region, an amount of conductive layers corresponding to the dummy memory string structures is greater than 10. An area of the dummy memory string structures in the first region is greater than an area of the dummy memory string structures in the second area under an identical unit area in a top view.