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. A capping film including an acid source is formed on the exposed surface areas 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 after removing the acid diffused regions of the second mask layer.