Abstract: Deep dephosphorization experiment was carried out in the laboratory by BaO-CaO-CaF₂ flux to obtain ultra-low phosphorus steel. The result showed that the P content of the tested steel could decrease from about 0.01% to 0.005% or even lower to 0.002% with a dephosphorization ratio of 50% by use of this kind of flux. It is found that the oxidizability of steel has significant effect on dephosphoriztion when using the BaO-CaO-CaF₂ flux. The phosphate capacity of the used system varied from 10¹⁸ to 10^19.35 in 1813~1893 K and increased with the increase of BaO content.