Methods of manufacturing a semiconductor device include forming integrated structures of polysilicon patterns and hard mask patterns on a substrate divided into at least an NMOS forming region and a PMOS forming region. A first preliminary insulating interlayer is formed on the integrated structures. A first polishing of the first preliminary insulating interlayer is performed until at least one upper surface of the hard mask patterns is exposed, to form a second preliminary insulating interlayer. The second preliminary insulating interlayer is etched until the upper surfaces of the hard mask patterns are exposed, to form a third preliminary insulating interlayer. A second polishing of the hard mask patterns and the third preliminary insulating interlayer is performed until the polysilicon patterns are exposed to form an insulating interlayer. The polysilicon patterns are removed to form an opening. A metal material is deposed to form a gate electrode pattern in the opening.