In contrast to uncoated substrate, a nonlinear relationship of phase shift with the thicknesses of the thin film makes the calculation of wavefront aberration complicated. A program is compiled to calculate the wavefront aberration of multilayer thin film produced by thickness nonuniformity. The physical thickness and the optical phase change on re°ection are considered. As an example, the wavefront aberration of the all-dielectric mirror is presented in ArF excimer lithography system with a typical thickness distribution. In addition, the wavefront errors of the thin film at wavelengths of 193 and 633 nm are compared in the one-piece and two-piece arrangements. Results show that the phase shift upon re°ection of the thin film produced by thickness nonuniformity is very sensitive to the incident angle, wavelength, and polarization.