Abstract: The effect of slag composition on theC content of ultra-low carbon steel at the end of smelting was investigated by the coexistence theory,dynamics analysis and experimental verification. A regression model of end-pointC content was established with temperature and slag composition in a temperature range from 1853 K to 1973 K. The results show that the Fe O activity is little affected by temperature,while it is mainly affected by slag composition. However,the decarburization kinetics is mainly affected by slag composition and temperature. The end-pointC content increases with increasing slag basicity,but it quickly decreases with increasing Fe O content in slag,so the end-point Fe O mass fraction in slag should be controlled in 12%-18%. In addition,the increment value ofC is less than 0. 01% with the Mg O mass fraction in slag from 7% to 13%. The results also show that the end-pointCmass fraction decreases with increasing temperature. Compared with industrial results,the average error rate of the regression model for the end-pointC content in the ultra-low carbon steel is ±15. 25%,and the hit rate toC content predicted by the regression model reaches 69. 19% with a control precision of ±0. 01% for heats of the experiment.