A method for fabricating a nanowire transistor is disclosed. First, a substrate is provided, and a stack structure is formed on the substrate, in which the stack structure includes a first semiconductor layer and a second semiconductor layer and the first semiconductor layer and the second semiconductor layer are made of different material. Next, a hard mask is formed on the stack structure and a first spacer adjacent to the hard mask, part of the stack structure is removed; a second spacer is formed adjacent to the first spacer and the stack structure; and a source/drain structure is formed adjacent to two sides of the second spacer.