Yttrium aluminum garnet （YAG） crystals are widely used for manufacturing solid-state lasers， and ultraprecision grinding is critical for machining hard and brittle material parts， such as YAG crystals. The investigation of microdeformation and brittle-to-ductile transition mechanisms of hard and brittle material-machined surfaces is necessary for ultraprecision grinding. Based on the elastic-plastic contact theory and indentation fracture mechanics， a model for predicting the critical depth of brittle-to-ductile transitions was established to achieve low-damage grinding of YAG crystals and obtain high-quality surfaces. The deformation process of the material surface under the action of a single abrasive scratch was analyzed， considering the elastic recovery of the material and the size effect of micromechanical properties. The critical depth of the brittle-to-ductile transition of the YAG crystal is 66.7 nm. The proposed prediction model of the critical depth of the brittle-to-ductile transition was verified by ultraprecision grinding of YAG crystals with grinding wheels of different grain sizes. In addition， the grit-cutting depths of different grain-size grinding wheels under the corresponding process conditions were calculated. The results show that when the grit-cutting depth is more extended than the critical depth of the brittle-to-ductile transition， the surface material of the YAG crystal is removed in a brittle manner， and the grinding surface is severely damaged. However， when the grit-cutting depth is less than the critical depth of the brittle-to-ductile transition， the grinding surface material is removed in a ductile manner， high-quality grinding surface can be obtained， and the machined surface roughness can reach 1 nm. The proposed model for predicting the critical depth of the brittle-to-ductile transition provides theoretical guidance for low-damage ultraprecision grinding of YAG crystals.