According to an exemplary embodiment, a method of forming a fin structure is provided. The method includes the following operations: etching a first dielectric layer to form at least one recess and a first core portion of a fin core; form an oxide layer as a shallow trench isolation layer in the recess; etching back the oxide layer to expose a portion of the fin core; and forming a fin shell to cover a sidewall of the exposed portion of the fin core.