A new method of measuring thickness of optical thin film with dispersive white-light spectral interferometry is proposed. By analyzing the recorded interference spectrum in the frequency domain and fitting the recorded channel spectrum to the theoretical value, the thin-film thickness can be determined. The thicknesses of several different thin film samples are calculated and the results show that this method can reach high precision with the error less than 1 nm. The new method can provide a simple and fast solution in calculating thickness of optical thin films, compared with traditional spectrophotometric and ellipsometric method, while still maintaining high accuracy.