Abstract: In the process of feeding RE wires in a mold, an amount of RE oxides enter into the molten slag, thus affecting and restricting the mold flux to serve metallurgical functions. Effect of RE oxides on the melting temperature, viscosity and crystallization temperature of mold flux was measured systemically. It was found that the melting temperature, viscosity and crystallization temperature of mold flux increase remarkably with increasing RE oxides as addition. If adding B2O3 and Li2O as new fluxes to the mold flux, the effect of RE oxides on the physicochemical properties of mold flux may be weakened remarkably. A theoretical basis is therefore available to develop further a new type of mold flux for RE steel, of which both the composition and physicochemical properties will be more reasonable.