Absorption coefficient and extinction coefficient for various commercial mold fluxes have been determined to quantify the radiative and the total heat transfer through the flux film in continuous casting mold. The absorption coefficient is found to be less than 1 000 m-1 for glassy specimens whereas the extinction coefficient is ca. 3 000-30 000 m-1 for crystalline ones. Comparison of observed with calculated radiative heat flux from the absorption coefficient has shown that gray gas approximation is valid for evaluating the radiative heat flux across the mold flux film. Numerical calculation for both radiative and conductive heat transfer has been carried out for a given total thickness of the flux film. Despite little difference in optical properties among various mold fluxes, the radiative heat flux through molten layer of the flux films for low carbon and ultra low carbon steel is found larger than that for medium carbon steel. This is due to increasing thickness ratio of molten layer to crystalline layer of flux films. Solidifying and crystallizing behavior of mold fluxes is a key factor to control the radiative heat transfer in continuous casting mold, accordingly.