A method of forming dense hole patterns of semiconductor devices includes: forming a plurality of first pillars on at least one lower hard mask layer disposed on a substrate; forming a spacer layer on the lower hard mask layer to form a plurality of second pillars respectively covering the first pillars, wherein a plurality of first holes are formed among the second pillars; etching the spacer layer to expose first portions of the lower hard mask layer via the first holes and expose top surfaces of the first pillars; removing the first pillars to form a plurality of second holes in the spacer layer to expose second portions of the lower hard mask layer; etching the first portions and the second portions of the lower hard mask layer at least until portions of the substrate are exposed; and removing remaining portions of the spacer layer.