The work aims to explore the surface formation mechanism of optical glass with different physical properties by cutting process and the effects of radial ultrasonic impact on the surface characteristics during rotary ultrasonic sawing (RUS). Firstly, the hardness and fracture toughness of two optical glasses, K9 and quartz, were compared through micro-indentation experiment, and the respective critical cutting depth were calculated simultaneously. Secondly, the finite element simulation of single grain cutting was carried out by LS-DYNA to figure out surface formation mechanism of optical glass with different physical qualities. Then, the influence of radial ultrasonic vibration on the surface characteristics of optical glass with different physical properties was analyzed by RUS and conditional sawing (CS) experiments. The fracture toughness of K9 and quartz glass was 5.232 MPa?m0.5 and 1.644 MPa?m0.5 respectively and the critical cutting depth was 4.288 μm and 0.012 μm respectively. The simulation results demonstrated that the maximum cutting depth of single grain was increased from 1.1767 μm to 8.001 μm by ultrasonic frequency impact in processing. From the cutting experiment results of quartz and K9 glass, for K9 glass, the roughness and peak-to-valley of the bottom of groove after RUS were significantly lower than that by conditional sawing, while for quartz glass, the decreasing was not obvious. Owing to the higher cutting depth of single grain by ultrasonic vibration, the material removal mode of K9 glass could be transformed into brittle fracture removal mode from partial plastic removal, which makes micro-chipping of surface. The surface roughness is improved by avoiding large area cracking and cracks in conditional machining. Attributed to the lower critical cutting depth of the quartz glass, the removal mode of the quartz glass in conventional sawing is brittle removal. Therefore, the effect of ultrasonic vibration is smaller, does not affect the surface roughness significantly.