To quantitatively measure the thickness d and refractiveindex n of the damage layer on sapphire substrate produced during chemical mechanical polishing,a spectral ellipsometric method is proposed. First, the change of spectral polarization state(i.e., amplitudes ratio and phases difference) of light(from 250 to 1650 nm) reflected on sapphire substrate is measured. Then, the thickness and refractive index of damaged layer are extracted by optical modeling and inversion of measured data. The d and n of substrates polished by Al,O% and SiO, abrasives are measured. Thed of the former fluctuates with the better result about 1.4 nm after 40 min, and the latter continues decreasing with the better result1 nm after 20 min. The n produced by two abrasives is both smaller than that of sapphire crystal. Meanwhile, the experiment and simulation analysis indicate that the change of phase difference is similar with d,which can be used to quickly evaluate the change of damage layer thickness because of no requirements for modelling and inversion. Therefore, the proposed method can be utilized tomonitor the machining process as a nondestructive optical way.