We present a simple,but highly sensitive,method for obtaining both the sign and magnitude of the nonlinear refractive index and the nonlinear absorption coefficient as a function of wavelength. It is shown theoretically that these paramters can be easily obtained from the linear and nonlinear thansmission spectra of the medium as well as the laser intensity distribution on the frequancy. This technique is demonstrated for ZnSe thin film at 77K and room temperature.