A patterning method and a method for fabricating a dual damascene opening are described, wherein the patterning method includes following steps. An organic layer, a silicon-containing mask layer and a patterned photoresist layer are formed on a material layer in sequence. The silicon-containing mask layer is removed using the patterned photoresist layer as a mask. A reactive gas is used for conducting an etching step so as to remove the organic layer with the silicon-containing mask layer as a mask, wherein the reactive gas contains no oxygen species. The material layer is removed using the organic layer as a mask, so that an opening is formed in the material layer. The organic layer is then removed.