Abstract: Based on the flow field in RH degasser, the metallurgical reaction thermodynamics and kinetics, a mathematical model was developed to investigate the decarburization process in RH degasser with side-bottom blowing. The numerical results indicate that the predicted results agree well with the experimental data. Under the condition of the same blowing gas flow rate, the decarburization rate in the first twenty minutes in RH with side-bottom blowing is greater than that in traditional RH. For decarburization in traditional RH, the decarburization in the inner site of the vacuum chamber accounts for the majority of the removed carbon in the first three seconds, while the decarburization rate at the surface of argon bubbles accounts for the majority of the removed carbon during three to one thousand seconds. For decarburization in RH with side-bottom blowing, the decarburization rate at the surface of argon bubbles accounts for the majority of the removed carbon in the first one thousand seconds, and decarburization rate at the surface of argon bubbles is as high as about two times of that in the inner site of the vacuum chamber. The production efficiency of ultra low carbon steel can be improved by increasing decarburization rate during final RH treatment.