A method of forming fine patterns of a semiconductor device according to a double patterning process that uses acid diffusion is provided. In this method, a plurality of first mask patterns are formed on a substrate so as to be separated from one another. A capping film including an acid source is formed on sidewalls and an upper surface of each of the plurality of first mask patterns. A second mask layer is formed on the capping films. A plurality of acid diffused regions are formed within the second mask layer by diffusing acid obtained from the acid source from the capping films into the second mask layer. A plurality of second mask patterns are formed of residual parts of the second mask layer which remain in the first spaces after removing the acid diffused regions of the second mask layer.