A FinFET device structure is provided. The FinFET device structure includes a fin structure formed over a substrate, and a gate structure formed over the fin structure. The FinFET device structure also includes an epitaxial source/drain (S/D) structure formed over the fin structure. A top surface and a sidewall of the fin structure are surrounded by the epitaxial S/D structure. A first distance between an outer surface of the epitaxial S/D structure and the sidewall of the fin structure is no less than a second distance between the outer surface of the epitaxial S/D structure and the top surface of the fin structure.