The disclosure relates to a method for making a grating. The method includes the following steps. First, a substrate is provided. Second, a photoresist film is formed on a surface of the substrate. Third, a nano-pattern is formed on the photoresist film by nano-imprint lithography. Fourth, the photoresist film is etched to form a patterned photoresist layer. Fifth, a mask layer is covered on the patterned photoresist layer and the surface of the substrate exposed to the patterned photoresist layer. Sixth, the patterned photoresist layer and the mask layer thereon are removed to form a patterned mask layer. Seventh, the substrate is etched through the patterned mask layer by reactive ion etching, wherein etching gases includes carbon tetrafluoride (CF4), sulfur hexafluoride (SF6) and argon (Ar2). Finally, the patterned mask layer is removed.