In most cases, hot metal dephosphorization slag is saturated withdicalcium-silicate that forms complete solid solution with tricalciumphosphate;the partition ratio of phosphorous between C2S andliquid slag is large. In order to utilize the effect of C2S phase for theimprovement of reaction efficiency, the optimum route of the slagcomposition change during dephosphorization should be clarified.The results of the fundamental experiments about the phosphorouspartition between C2S and liquid slag from the viewpoints of equilibriumand kinetics are reported.