Abstract: In order to obtain ultra-smooth optical surface in ion beam process, the process methods for reducing surface roughness are introduced and validated by experiments. Optical material is typical hard and brittle material, whose surface roughness evolves complicated variation in process. As the final precision machining, ion beam figuring can correct figure error and improve optical surface roughness. Based on the evolvement mechanism of surface roughness in ion beam process, the methods of obliquely incidence figuring and coating sacrificial layer technology were investigated. The surface roughness was measured by atomic force microscope (AFM). Experimental results indicate that the surface roughness was reduced from 0.67nm RMS to 0.38nm RMS when figuring fused silica sample was processed at oblique incidence angle of 45. Similarly, the surface roughness was reduced from 0.81nm RMS to 0.28nm RMS after removing sacrificial layer. The methods of obliquely incidence figuring and sacrificial layer figuring technology can effectively improve surface roughness in ion beam figuring.