A three-dimensional (3D) semiconductor device includes first interlayer dielectric layers and word lines that are alternately stacked on a substrate; select lines formed on the first interlayer dielectric layers and the word lines; etch stop patterns formed on the select lines to contact the select lines; channel holes formed to pass through the select lines, the first interlayer dielectric layers, and the word lines; channel layers formed on surfaces of the channel holes; insulating layers formed in the channel holes, the insulating layers having an upper surface that is lower than upper surfaces of the etch stop patterns; impurity-doped layers formed in channel holes on upper surface of the insulating layers; and a second interlayer dielectric layer formed over the etch stop patterns and the impurity-doped layers.