By using femtosecond laser assisted etching technology， a grating structure with adjustable period， duty cycle and height is achieved on sapphire surface. It solves the problem of poor surface quality in femtosecond laser processing of hard and brittle materials， such as low processing accuracy caused by debris accumulation， and difficulty in preparing deep structures. The roughness of the sapphire grating structure was reduced from 78 nm （after direct laser writing） to 7 nm （after dry etching）， and the sapphire microstructures with a period of 800 nm and a aspect ratio of 4 were fabricated. The femtosecond laser assisted etching technology can prepare a high smoothness grating on sapphire surface， and the result proves that this technology can significantly improve the diffraction efficiency of each diffraction order.