A method of fabricating a semiconductor device includes forming a cut-off region in at least one mandrel line among a plurality of mandrel lines, conformally forming a spacer material layer in the plurality of mandrel lines and a non-mandrel area and forming a cut spacer in the cut-off region and depositing a gap-fill material such that a cut block is formed on a portion of the non-mandrel area and a concave portion of the cut spacer is filled.