Abstract: A coupled thermodynamic model of inclusion precipitation and solute elements microsegregation during solidification of 253MA steel was established on the base of the reaction equilibrium principle of multiphase and polycomponent， and the redistribution theory of solute elements in the interface of liquid and solid during solidification. The accuracy of this model was validated through the commercial test and scanning electron microscope (SEM) analysis of inclusions formed in the samples. Under the rated condition， the inclusions in 253MA steel are mainly Ce2O3， Ce2O2S， Ce3S4， CeS， CeN， SiO2 and MnS. And the precipitation principles of these inclusions are obtained by this model. When oxygen content in 253MA steel is higher 0006%， Ce2O3 can precipitate if the ratio ［%O］ / ［%S］ is greater than 1. When oxygen content is lower than 0. 006%， Ce2O3 can precipitate if the ratio is greater than 2. When the oxygen content is lower than 00046%， Ce3S4 precipitates and CeN disappears if ［%O］+［%S］>0009; CeS exists in steel when ［%O］/［%S］<1 and transfers to Ce3S4 with the increase of the sulfur content. When the oxygen content is higher than 00046%， CeN can precipitate if 2［%O］+［%S］<0014.

Key words: heatresistance steel, rare earth steel, inclusions, cleanliness, thermodynamic model