Compared to conventional grinding, axial ultrasonic vibration-assisted peripheral grinding (AUPG) has advantages in terms of the improved grinding quality, higher efficiency, as well as lowered brittle damages. However, the present studies on material removal mechanism of AUPG are still not sufficient which thereafter limit its application potential. This paper aims to investigate the material removal mechanism of Zerodur and ULE through scratching by using a Vickers indenter under two conditions, with (ultrasonic vibration-assisted scratching (UVS)) and without (conventional scratching (CS)) axial ultrasonic vibration-assisted scratching while the other scratching parameters are identical. The indenter's kinematic characteristic, scratched morphology, scratched groove dimensions and critical depth of cut are compared in between UVS and CS. The experimental results indicate that the dynamic contact length between indenter and workpiece for UVS is remarkable longer than that for CS, which is helpful to promote the crack interference and hence to increase the proportion of ductile material removal mode, resulting in the diminishment of brittle fracture size in UVS. The propagation direction of median cracks in CS are relatively consistent, while in UVS the periodic varied contact zone and contact force in between the indenter and workpiece could promote the cracks propagating in different orientations, and then to improve the material removals rate in UVS. Moreover, both the critical brittle-ductile transition depth of cut and critical brittle depth of cut in UVS are bigger than that in CS, meaning the material removed in UVS with a bigger ductile ratio leading to a decreased brittle damage depth. In addition, the different material removal behaviours between Zerodur and ULE are also investigated. This fundamental work lays a theoretical foundation for the technological development and broad application of hard/brittle material oriented AUPG, as well as poses a meaningful guidance for the ultrasonic vibration assisted grinding on low-expansion optical glasses. © 2020