In the method of forming a mask structure, a first mask is formed on a substrate where the first mask includes a first mask pattern having a plurality of mask pattern portions having openings therebetween and a second mask pattern having a corner portion of which an inner side wall that is curved. A sacrificial layer is formed on the first mask. A hard mask layer is formed on the sacrificial layer. After the hard mask layer is partially removed until the sacrificial layer adjacent to the corner portion is exposed, a second mask is formed from the hard mask layer remaining in the space after removing the sacrificial layer. A minute pattern having a fine structure may be easily formed on the substrate.