Desulfurization of molten steel was carried out in a high frequency induction furnace at 1550℃. Rates of desulfurization by CaO powder were measured with varing the amount of aluminum as a reducing agent, and were investigated and compared under conditions of the powder injection and top powder. Results showed that the desulfurization rate by the powder injection method was considerably faster than that by the top powder method. Also the rate was increased with increasing the amount of aluminum added. It is interesting to note that product CaS as well as unreacted CaO powder floats immediately after the injection, leaving CaS and CaO free melt. Rate equation for desulfurization was developed to be the first order with respect to sulfur activity when dissolved aluminum and oxygen remain constant, and the desulfurization mechanism was proposed.