Based on the dispersion equations，“Downhill Simplex” method is utilized to measure the thickness and optical constants of thin film by fitting the curve of measured transmission spectrum. TiO2, Si3N4 thin films deposited by electron gun evaporation and reactive magnetron sputtering respectively are measured with Cauthy formula and ITO film deposited by electron gun evaporation with Drude formula. Experiments show that the results got by this method and by α-step apparatus agree well. No initial input is necessary for this simple method to realize the global optimization. With various dispersion formula, it is also suitable for many kinds of films with thinner thickness, which will be useful in thin film optics, microelectronic and micro-optical electro-mechanical system (MOEMS).