A method for forming FinFETs includes, sequentially, providing a substrate; forming a plurality of fins on a surface of the substrate; forming a gate structure overlying on at least one of the plurality of fins; forming a barrier layer covering top and side surfaces of the gate structures, and top and side surfaces of the plurality of fins; performing a radical oxidation process to convert a top portion of the barrier layer to a passive layer to form a remaining barrier layer and to cause the top surfaces of the fins to be flat after subsequent etching processes; performing an etch-back process on the passive layer to form passive sidewalls on side surfaces of the portions of the remaining barrier on the side surfaces of the fins; and removing portions of the remaining barrier layer on the top surfaces of the fins by a wet etching process using the passive sidewalls as an etching mask.