This invention discloses a method of forming a prism sheet. The method comprises: providing a substrate having a major light input surface and a major light output surface opposite to the major light input surface; forming a prismatic structure on the major light output surface of the substrate; and forming an uneven structure on the major light input surface of the substrate, wherein forming the uneven structure on the major light input surface of the substrate comprises: providing a hard tool having a smoothly-curved shape such that the penetrating width of the hard tool increases as the penetrating depth of the hard tool increases; penetrating the hard tool into a mold and repeatedly moving the hard tool up and down to form a plurality of smoothly-curved concave shapes along a first path on the mold; and using the cut surface of the mold to emboss a thin film on the second path corresponding to the first path on the major light input surface of the substrate to form a plurality of convex shapes one-to-one corresponding to the plurality of smoothly-curved concave shapes on the second path on the major light input surface of the substrate; wherein the hard tool is not pulled away from the uncut surface of the mold so as to form the cut mold for embossing the thin film on the major light input surface of the substrate such that there is no space between each two adjacent convex shapes along the second path on the major light input surface of the substrate to maximize the uneven optical diffusing area on the second path on the major light input surface of the substrate.