Controlling inclusion composition,from the point of view of thermodynamics,only explains the probability and limit of reaction.However,kinetics makes the nucleation and the velocity of growth of inclusions clear,and these kinetic factors arc very important to the quality of slab.The basic kinetic theory of unreacted core model was used to build the mathematical model for the growth of inclusions and the concerned software was developed through Visual Basic 6.0.The time that different radius inclusions attain saturation was calculated to determine the controlling step of reaction between steel and inclusions.The time for the growth of inclusion obtained from the model was in good agreement with the data measured by Japanese Okuyama G,which indicated that the model is reasonable.