A memory device includes a substrate, first semiconductor layers and second semiconductor layers alternately stacked over the substrate, a first gate structure and a second gate structure crossing the first semiconductor layers and the second semiconductor layers, a first via and a second via over the first gate structure and the second gate structure, and a first word line and a second word line over the first via and the second via. Along a lengthwise direction of the first and second gate structures, a width of the first semiconductor layers is narrower than a width of the second semiconductor layers.