A mathematical model has been developed to predict the dissolution rate of added fluxes such as lime and dolomite as a sub-model of an overall kinetic model of top-blown oxygen steelmaking process. The calculation of dissolution rates included dimensionless analysis technique as well as the evolution of gas generated due to decarburization reaction during the blowing process. The progress of flux dissolution and the amount of slag generated were predicted as a function of saturation concentration of CaO and MgO in the slag, CO gas flow rate and the physical properties of slag throughout the blow. The results from the model were consistent with the plant data from the study of Cicutti et al. The influences of variations in solid particle size and flux addition rate on flux dissolution were also investigated. It has been shown in this study that the amount of flux dissolved in oxygen steelmaking process is accelerated if the addition rate of flux is increased or if the size of flux particle is decreased.