A method for estimating film thickness in CMP includes the following operations. A substrate with a film formed thereon is disposed over a polishing pad with a slurry dispensed between the film and the polishing pad. A CMP operation is performed to reduce a thickness of the film. An in-situ electrochemical impedance spectroscopy (EIS) measurement is performed during the CMP operation by an EIS device to estimate the thickness of the film real-time. The CMP operation is ended when the estimated thickness of the film obtained from the fit parameters of the first equivalent electrical circuit model reaches a target thickness.