The crack nucleation and propagation process in ultrasonic grinding engineering ceramics are studied based on indentation fracture mechanics. The study shows ultrasonic grinding can be considered as continuous loading and unloading process in view of a single grain. The loading makes cracks form and preliminarily propagate; while the unloading makes them propagate further and makes the direction of crack propagation offset to the workpiece surface, not so far as to extend to the material interior. In common grinding cracks have extended to the material interior, resulting in deep subsurface crack in the subsurface layers. In this paper, the ground surface of small crack damage can be obtained with large grinding depth in ultrasonic grinding compared with common grinding under the same conditions.