The ultimate value of surface roughness and its characteristic for the polishing process was investigated in this thesis. To find out the nature of ultimate surface roughness by means of the hydrodynamic polishing process and can be used for all polishing method. A preliminary mathematical model that was proposed to explain the ultimate surface roughness proceedings and a series of experiments was planed to verify suitable of this model. Starting with the rule for ultimate surface roughness happened. The assumption that abrasive particle at roughness peak and valley machining capability differ less than one atom, the ultimate surface roughness be attained. We can propose the mathematical model of ultimate surface roughness based on this rule and the previous study of hydrodynamic polishing process. Following by useing the computer simulation to help us exploring ultimate surface roughness characteristic and testing experiments fit for the forecasting. Finally, we explain reasons that cause the experiment results not agree with the model anticipation and propose better lubrication condition to polish the optimum surface.