The reference wavelength method is used for a two-color pyrometer and, with the reference wavelength method, an analytical formula of the ratio temperature for the two-color pyrometer is derived. For one channel of the two-color pyrometer, with a triangular spectral response of 0.1-micro m FWHM and 2.0-micro m peak wavelength, the effective wavelength and the correction factors with several reference wavelengths are determined. By fitting the curves of the effective wavelength and the correction factor to simple functional forms of temperature, the radiance errors for both cases are calculated. Also, it is found that the correction factor determined in a single-color pyrometer can be used directly in one channel of the two-color pyrometer without additional calculation.