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