Abstract: Al₂O₃-SiC-C castables for iron runner were prepared with brown corundum as aggregates and with silicon carbide, white corundum powder, alumina micropowder, sodium hexametaphosphate, etc., as auxiliary materials. Effect of sodium hexametaphosphate addition amount (0.05%-0.20%, mass fraction) on the compactness, mechanical properties and thermodynamic properties of the castables was investigated. The results show that after drying at 110 ℃ and firing at 1 450 ℃, the bulk density, compressive strength and flexural strength of the castable samples increased first and then decreased with increasing addition of sodium hexametaphosphate, while the apparent porosity decreased first and then increased. The linear expansion coefficient of the sample with 0.10% sodium hexametaphosphate was larger than that of the sample with 0.15wt% sodium hexametaphosphate. When the testing temperature was lower than 1 150 ℃, the thermal conductivity of the sample increased first and then decreased with increasing addition of sodium hexametaphosphate. Considering various factors, the optimal addition amount of sodium hexametaphosphate was 0.10%-0.15%.