A method of forming a semiconductor device includes forming first sacrificial patterns on a lower structure, forming first remaining mask layers having a “U” shape between the first sacrificial patterns to be in contact with the first sacrificial patterns, forming first remaining mask patterns by pattering the first remaining mask layers, each of the first remaining mask patterns including a horizontal portion, parallel to an upper surface of the lower structure, and a vertical portion, perpendicular to the upper surface of the lower structure, forming second mask patterns spaced apart from the vertical portions of the first remaining mask patterns, removing the first sacrificial patterns remaining after forming the second mask patterns, and forming first mask patterns by etching the horizontal portions of the first remaining mask patterns.