As an important factor in the moderation of heat transfer in continuous casting mold, mold flux has been researched widely. However, the study of the effect of solid mold flux on radiative heat transfer has not been conducted widely. By using an infrared radiation emitter, which was developed at Carnegie Mellon University, a radiative heat flux was applied to a copper mold to simulate the heat transfer phenomena in continuous casting. The effect of adding a thin slag disc on top of copper mold on radiative heat transfer has been analyzed. It was found that the presence of mold flux enhanced radiative heat transfer rate. The effect of full crystallization of a slag disc was to reduce the heat transfer rate by 20% compared with a completely glassy sample. The specific effect of full crystalline and glassy parts of the mold flux on radiative heat transfer, and the influence of their properties on heat transfer rate was discussed in this paper.