A method for manufacturing a semiconductor device includes forming a semiconductor strip over a substrate. The semiconductor strip includes a first semiconductor stack and a second semiconductor stack over the first semiconductor stack. A dummy gate stack is formed to cross the semiconductor strip. The dummy gate stack is replaced with a first metal gate stack and a second metal gate stack. The first metal gate stack is in contact with the first semiconductor layer of the first semiconductor stack and the second metal gate stack is in contact with the first semiconductor layer of the second semiconductor stack.