Abstract: A mathematical model was developed to describe the relationship between the decarburization rate in bath and the operation parameters in the UHP electric arc furnace with hot metal charged, based on the reaction kinetic and mass balance. The model can be used to accurately describe the variation of carbon content in the bath and dynamically control the carbon content at the melting end. The calculated results were in good agreement with the production data of an 100 t UHP electric arc furnace. The simulated results showed that the decarburization rate is proportional to the intensity of oxygen supply, the square root of carbon content and the carbon content in bath in the ranges of high, medium and low carbon content, respectively. An optimum melting effect can be obtained if operating in accordance to the most economical oxygen flowrate curve, i.e., in such a jetting way that the oxygen flowrate should be decreased with decreasing carbon content in molten steel.