A method of forming a nanowire includes providing a substrate. The substrate is etched to form at least one fin. Subsequently, a first epitaxial layer is formed on an upper portion of the fin. Later, an undercut is formed on a middle portion the fin. A second epitaxial layer is formed to fill into the undercut. Finally, the fin, the first epitaxial layer and the second epitaxial layer are oxidized to condense the first epitaxial layer and the second epitaxial layer into a germanium-containing nanowire.