Abstract: A method monitors a thickness of a subject film deposited on an underlying structure, the underlying structure contains at least one thin film formed on a substrate. The method includes determining thickness data of the underlying structure and storing the thickness data of the underlying structure in a thickness memory; measuring profile of optical spectrum of the subject film on the underlying structure; reading the thickness data of the underlying from the thickness memory; calculating theoretical profiles of the optimal spectrum of the subject film based upon corresponding candidate film thicknesses of the subject film and the thickness data of the underlying structure; and searching a theoretical profile of the subject film, which is closest to the measured profile of optical spectrum of the subject film so as to determine a thickness of the subject film.