A novel thin film thickness measurement method is proposed. In the method, a transition layer which is thick enough for interference is used as a substrate. So, the thickness of the deposited layer can be accurately measured by measuring the shift of an interference peak before and after deposition. Because the thickness of the transition layer has already induced interference, the interference can be changed even if the newly deposited layer is very thin. According to the change of the interference in transmittance or reflectance spectra, the thickness of the layer can be determined accurately and easily with a measurement limit of 1 nm at least. This method is simple and fast in operation. It is very suitable for the on-line detection and real-time monitoring in coating industries, particularly for the thickness measurement of weak absorption ultra-thin films.