The existing research about ductile grinding of fused silica glass was mainly focused on how to carry out ductile regime material removal for generating very “smoothed” surface and investigate the machining-induced damage in the grinding in order to reduce or eliminate the subsurface damage.The brittle/ductile transition behavior of optical glass materials and the wear of diamond wheel are the most important factors for ductile grinding of optical glass.In this paper,the critical brittle/ductile depth,the influence factors on brittle/ductile transition behavior,the wear of diamond grits in diamond grinding of ultra pure fused silica(UPFS) are investigated by means of micro/nano indentation technique,as well as single grit diamond grinding on an ultra-stiff machine tool,Tetraform “C”.The single grit grinding processes are in-process monitored using acoustic emission(AE) and force dynamometer simultaneously.The wear of diamond grits,morphology and subsurface integrity of the machined groves are examined with atomic force microscope(AFM) and scanning electron microscope(SEM).The critical brittle/ductile depth of more than 0.5 μm is achieved.When compared to the using roof-like grits,by using pyramidal diamonds leads to higher critical depths of scratch with identical grinding parameters.However,the influence of grit shapes on the critical depth is not significant as supposed.The grinding force increased linearly with depth of cut in the ductile removal regime,but in brittle removal regime,there are large fluctuations instead of forces increase.The SEM photographs of the cross-section profile show that the median cracks dominate the crack patterns beneath the single grooves.Furthermore,The SEM photographs show multi worn patterns of diamond grits,indicating an inhomogeneous wear mechanism of diamond grits in grinding of fused silica with diamond grinding wheels.The proposed research provides the basal technical theory for improving the ultra-precision grinding of UPFS. The existing research about ductile grinding of fused silica glass was mainly focused on how to carry out ductile regime material removal for generating very “smoothed ” surface and investigate the machining-induced damage in the grinding in order to reduce or eliminate the subsurface damage The brittle / ductile transition behavior of optical glass materials and the wear of diamond wheel are the most important factors for ductile grinding of optical glass. In this paper, the critical brittle / ductile depth, the influence factors on brittle / ductile transition behavior, the wear of diamond grits in diamond grinding of ultra pure fused silica (UPFS) are investigated by means of micro / nano indentation technique, as well as single grit diamond grinding on an ultra-stiff machine tool, Tetraform “C”. single grit grinding processes are in-process monitored using acoustic emission (AE) and force dynamometer simultaneously. The wear of diamond grits, morphology and subsurface integrity of the machined groves are examined with atomic force microscope (AFM) and scanning electron microscope (SEM). The critical brittle / ductile depth of more than 0.5 μm is achieved. When compared to the using roof-like grits, by using pyramidal diamonds leads to higher critical depths of scratch with identical grinding parameters. However, the influence of grit shapes on the critical depth is not significant as the. The grinding force increased linearly with depth of cut in the ductile removal regime, but but in brittle removal regime, there are very large fluctuations instead of forces increase.The SEM photographs of the median-cracks dominate the crack patterns beneath the single grooves. Morerther, The SEM photographs show multi-worn patterns of diamond grits, indicating an inhomogeneous wear mechanism of diamond grits in grinding of fused silica with diamond grinding wheels. The proposed research provides the basal technical theory for improving the ultra-precision grinding of U. PFS.