The present invention discloses a method of forming an optical sheet. The method comprises: providing a mold having a first surface; forming a plurality of first concave shapes on the first surface of the mold such that the first surface of the mold is changed to a second surface of the mold; forming a plurality of second shapes on the plurality of first concave shapes such that the second surface of the mold is changed to a third surface of the mold; and using the third surface of the mold to emboss a film on a substrate to form a composite structure corresponding to the combination of the plurality of first concave shapes and the plurality of second shapes.