Method for forming Fin field effect transistor (FinFET) device structure

Methods for forming the fin field effect transistor (FinFET) device structure are provided. The method includes forming first fin structures and second fin structures on a first region and a second region of a substrate, respectively, and a number of the first fin structures is greater than a number of the second fin structures. The method also includes forming a sacrificial layer on the first fin structures and the second fin structures and performing an etching process to the sacrificial layer to form an isolation structure on the substrate.