A method of forming a mask pattern for fabricating a semiconductor device. A first region and a second region, having an intersecting third region, are defined in the semiconductor substrate. An inorganic mask layer is etched in the first region to a predetermined thickness, and etched in the second region to another predetermined thickness. While the inorganic mask layer is etched in the first and second region, an organic mask layer is exposed in the third region. The organic mask layer exposed in the third region is removed to form a mask pattern. Consequently, double exposure is performed using the organic mask layer and the inorganic mask layer, so that a fine feature size that closely follows a desired layout can be formed, damage to the organic mask layer by ashing is prevented, and adhesiveness between the organic mask layer and the inorganic mask layer can be improved.