Abstract: A thermodynamic model was developed quantitatively to analyze the behavior of the RE element cerium in the molten steel for heavy rail, in which the existence state of Ce and constituents of inclusions can be forecasted under different cleanliness. The results showed that in the molten steel the increasing mass fractions of initial oxygen and sulfur benefit the precipitation of Ce oxide/Ce oxysulfide and that of Ce sulfide/Ce oxysulfide, respectively. The necessary conditions for Ce2O3 and Ce2S3 precipitations at 1783 K are that the activity ratio of oxygen to sulfur is greater than or equal to 0.19 and less than or equal to 0.39, respectively. The necessary condition for transforming CeS into Ce2O2S is that the activity ratio of oxygen to sulfur is 0.53-0.96. With the cleanliness improved, the mass fraction of Ce solute increases gradually.