The disclosure relates to a method for making a grating. The method includes the following steps. First, a substrate is provided. Second, a patterned mask layer is formed on a surface of the substrate. Third, the substrate with the patterned mask layer is placed in a microwave plasma system. Fourth, a plurality of etching gases are guided into the microwave plasma system simultaneously to etch the substrate through three stages. The etching gas includes carbon tetrafluoride (CF4), argon (Ar2), and sulfur hexafluoride (SF6). Finally, the patterned mask layer is removed.