Laboratory experiments have been performed to make clear heat transfer behavior in the mold of continuous casting of steel. The overall thermal resistance of a parallel-sided plate filled with mold flux has been measured. The interfacial thermal resistance between the mold and flux film interface and the thermal conductivity of the mold flux are analyzed quantitatively. An interfacial thermal resistance corresponding to the air gap of 20-50 μm is observed in the case of solid mold flux. When the temperature of the mold surface exceeds solidification temperature of the flux, the interfacial thermal resistance disappears together with disappearance of air gaps. The crystallization of the mold flux inhibits the radiative heat transfer that is equivalent to 20% in the total heat flux. The thermal conductivity of the mold flux depends on the size of silicate ions. Large size of silicate ion promotes the conductive heat transfer.