Abstract: The composition of mould fluxes was designed according to the principle of regressive orthogonal design, and the effect of RE_xO_y on viscosity and melting temperature of mould fluxes have been investigated. The results indicate that at the condition of basicity more than 0.85, the melting temperature and viscosity of mould fluxes would be increased with the increasing of RE_xO_y in mould fluxes, which leads to the decreasing of dissolving capacity of in mould fluxes. At the condition of basicity less than 0.80, the melting temperature and viscosity of mould fluxes would be decreased with the increasing of RE_xO_y in mould fluxes.