Variation of the surface removal rate with down pressure, pad asperity, and relative velocity in chemical-mechanical polishing (CMP) was studied. A phenomenological model for contact of pad asperities with abrasive particles and wafer is described. The functional dependence of the polishing rate on pressure and velocity was found to be related to the distribution of pad asperities. Simple argument based on the mechanical contact theory suggests that the linear variation predicted by Preston's equation follows when the pad asperities have a random distribution. The sublinear dependence of the removal rate, however, is obtained when the pad asperities have a wavy distribution. The use of the statistical least-square method is suggested as a way to determine the relationship between the removal rate and pressure for different industrial pads used in CMP processes.