In a method of forming patterns of a semiconductor device, a to-be-etched layer is formed on a semiconductor substrate. First etch mask patterns are formed over the to-be-etched layer. An auxiliary layer is formed on the first etch mask patterns and the to-be-etched layer. The auxiliary layer is thicker on upper sidewalls of the first etch mask patterns than on lower sidewalls thereof. Second etch mask patterns are formed in concave portions of the auxiliary layer. The auxiliary layer between the first and second etch mask patterns is removed. The to-be-etched layer is patterned using the first and second etch mask patterns as an etch mask.