A method for fabricating a fin field effect transistor (FinFET) is provided. The method includes: patterning a substrate to form a plurality of trenches in the substrate and at least one semiconductor fin between the trenches; forming a plurality of insulators in the trenches; forming a patterned photoresist on the insulators, wherein sidewalls of the semiconductor fin are partially covered by the patterned photoresist, and at least one area of the sidewalls is exposed by the patterned photoresist; by using the patterned photoresist as a mask, partially removing the semiconductor fin from the at least one area of the sidewalls exposed by the patterned photoresist so as to form at least one recess on the sidewalls of the semiconductor fin; removing the patterned photoresist after forming the at least one recess; and forming a gate stack to partially cover the semiconductor fin and the insulators.