Abstract(#br)Monocrystalline sapphire, which is widely used in micro-machining industry, can be seriously affected by fracture on its surface quality in micro-slot-grinding. In this paper, a new fracture size predicting model considering different crystal orientations is established. Experiments about micro-slot-grinding of single crystal sapphire considering three different orientations, (11 2 ¯ 0), (0001) and (1 1 ¯ 02), were conducted to find the different fracture behaviors among orientations and the exact changing rules of surface fracture size. Basing on the observation of experimental results, it is found that the fracture size is increasing with the ascending of feeding rate speed and cutting depth. Furthermore, (11 2 ¯ 0) orientation owns... the biggest fracture size, fracture on (0001) orientation is smaller than (11 2 ¯ 0) orientation, but all these two orientations have a much bigger fracture size than (1 1 ¯ 02) orientation, which makes (1 1 ¯ 02) the easiest machining orientation. Different fracture behaviors and cracks’ dimensions are also observed and measured. The fracture on (11 2 ¯ 0) orientation is concessive wavy structure, on (0001) orientation is lamellar tearing structure, on (1 1 ¯ 02) orientation is continuous small single fracture. The dimension of subsurface crack is obtained and measured through a corrosion procedure. The size relationship between subsurface crack and machining parameter is revealed, different internal topographies of subsurface crack are observed: on (11 2 ¯ 0) orientation is rectangle, on (0001) and (1 1 ¯ 02) orientations are triangle and rhombus respectively. Micro-structure and phase structure of subsurfaces of (11 2 ¯ 0) and (1 1 ¯ 02) orientations are revealed by scanning electron microscope (SEM) and energy dispersive spectrometry (EDS). Model verification work also has been done, and the comparison results indicate that the experimental results fit well with the model predicting outcomes. The knowledge this study presents is significant to the precision micro-machining of sapphire.