The influences of Al2O3/Na2O ratio and MnO content on high-temperature properties,such as melting,crystallization,heat transfer,and viscosity of mold flux for casting peritectic steel,have been investigated.The results show that the melting temperatures of mold flux decrease,whereas the ratio of crystalline layers increases with the decrease in Al2O3/Na2O ratio and increase in MnO content.The average response temperatures of the three mold fluxes decrease from 566,525,to 512℃,respectively,which indicates that the heat transfer controlling ability of mold flux is promoted due to the increase in crystallization ability and addition of transition metal oxide MnO.Furthermore,the viscosity-temperature curves suggest that the viscosity at 1300℃decreases,but the break temperature increases with the reduction in Al2O3/Na2O and addition of MnO.