A semiconductor device includes a substrate, a channel structure, and a gate structure. The channel structure is over the substrate and extends along a first direction, in which the channel structure has plurality of first portions and plurality of second portions alternately stacked, and a width of the first portions is smaller than that of the second portions in a second direction different from the first direction. The gate structure is disposed over the substrate and crossing the channel structure along the second direction, in which the gate structure is in contact with the first portions and the second portions.