A method for making grating is provided. The method includes following steps. A substrate is provided. A mask layer is located on the substrate. The mask layer is patterned, and a number of bar-shaped protruding structures are formed on a surface of the mask layer, a slot is defined between each of two adjacent protruding structures of the number of protruding structures to expose a portion of the substrate. The protruding structures are etched so that each of two adjacent protruding structures begin to slant face to face until they are contacting each other. The exposed portion of the substrate is etched through the slot. The mask layer is removed.