Abstract: Laboratory study was carried out on deep dephosphorization of liquid steel by BaO-based fluxes at 1600℃ to achieve ultra low phosphorus level in the steel. A dynamic model of dephosphorization of liquid steel by BaO-based fluxes was established.According to the model, the controlling step of dephosphorization by BaO-based fluxes was the mass-transfer of phosphorus in steel phase under the condition of low phosphorus liquid steel. It is estimated that the apparent rate constant of dephosphorization by BaObased fluxes was 0.4×10^-3-8.2×10^-3 g/(cm².s) and the mass-transfer coefficient of phosphorus in bulk steel was 0.025-0.04 cm/s.