Abstract: Grain depth-of-cut has a significant impact on surface- or subsurface-damage (SSD) of the ultra-ground parts of hard and brittle materials. Thus, to better simulate and predict the depth-of-cut, the differences between prediction and result are analyzed, mainly about why prediction is smaller than result. The model is fixed according to deviation analysis on grain numbers and test result of single-grain scratch. However, the fixed model, though more accurate, still could not meet the results. Some other factors that might influence the simulation are put forward, such as radius of cutting and minimum thickness of chip, to provide reference for further development and promote the optimization of grinding parameters.