A method for forming a FinFET device structure includes forming a first fin structure in a core region of a substrate and a second fin structure in an input/output region of the substrate with a fin top layer and a hard mask layer over the fin structures. The method also includes forming a dummy oxide layer across the fin structures. The method also includes forming a dummy gate structure over the dummy oxide layer. The method also includes removing the dummy gate structure over fin structures. The method also includes removing the dummy oxide layer and trimming the fin structures. The method also includes forming first and second oxide layers across the first and second fin structures. The method also includes forming first and second gate structures over the first and second oxide layers across the first and second fin structures.