A method for fabricating a FinFET structure comprises providing a semiconductor substrate; forming a hard mask layer on the semiconductor substrate; forming a dummy gate structure having a dummy gate, a first sidewall spacer and a second sidewall spacer; removing the dummy gate to form a first trench; forming first sub-fins in the semiconductor substrate under the hard mask layer in the first trench; forming a first metal gate structure in the first trench; removing the first sidewall spacer to form a second trench; forming second sub-fins in the semiconductor substrate under the hard mask layer in the second trench; forming a second metal gate structure in the second trench; removing the second sidewall spacer to form a third trench; forming third sub-fins in the semiconductor substrate under the hard mask layer in the third trench; and forming a third metal gate structure in the third trench.