A semiconductor device includes a polycrystalline semiconductor layer on a substrate, first and second stacks on the polycrystalline semiconductor layer, the first and second stacks extending in a first direction, a separation trench between the first and second stacks and extending in the first direction, the separation trench separating the first and second stacks in a second direction crossing the first direction, and vertical channel structures vertically passing through each of the first and second stacks, wherein the polycrystalline semiconductor layer includes a first grain region and a second grain region in contact with each other, the first and second grain region being adjacent to each other along the second direction, and wherein each of the first and second grain regions includes a plurality of crystal grains, each crystal grain having a longitudinal axis parallel to the second direction.