In a method of forming a nanowire in a semiconductor device, a trench is formed by partially etching a bulk semiconductor substrate. An insulation layer pattern is formed on the substrate to fill up the trench. The insulation layer pattern covers a first region of the substrate where the nanowire is formed, and additionally covers a second region of the substrate connected to the first region. An opening is formed by etching an exposed portion of the substrate by the insulation layer pattern. A spacer is formed on sidewalls of the opening and the insulation layer pattern. The nanowire connected to the second region is formed by anisotropically etching a portion of the substrate exposed by the opening until a portion of the insulation layer pattern formed in the trench is exposed.