An analytic model of the material removal rate is proposed for chemical mechanical planarization (CMP). The effects of the applied pressure and the polishing velocity between the wafer surface and the pad surface are derived considering the chemical reaction as well as the mechanical bear-and-shear process. The mechanism of microscopic material removal is presented. The material removal rate is found less linearly correlated to the pressure and relative velocity between the pad and the wafer, which was predicted by the frequently cited empirical Preston equation obtained from glass polishing. [I. F. W. Preston, J. Soc. Glass. Technol, 11, 214 (1927)]. It provides the analytical modification of the Preston equation in the dielectrics CMP. The experimental results of authors and independent researchers are discussed.