Abstract: In consideration of the equilibrium of C and O elements, a mathematical model was established for decarburization in RH refining. The principle and process of establishing the model were illustrated according to 210 t RH refining for ultra-low carbon steel. A comparison between the modeling results and measuring data shows that the prediction of the mathematical model agrees well with industrial practice. There exists the nonuniform characteristic of carbon in liquid steel. The position with the minimum mass fraction of carbon is at the free surface in the vacuum chamber above the descending snorkel, while the position with the maximum mass fraction of carbon is at the interface between slag and liquid steel on the right side of the ascending snorkel. The difference between the maximum and minimum values is 0.002 5% after 20 min circulating.