Abstract: Based on a dynamical research on deoxidation and desulfurization of molten steel with calcium treatment, it is found that when solid calcium particles are fed into molten steel, some dissolve and the others become bubbles reacting with oxygen and sulfur. The bigger the diameter of the calcium particle is, the bigger the residenee time and average flotation speed of the bubble in molten steel are, and the less the mass transfer coefficients of deoxidation and desulfurization are. The residence time and flotation speed have hardly relationship with temperature in the steelmaking temperature range, but the mass transfer coefficient becomes large with increasing temperature. With increasing the contents of oxygen and sulfur in molten steel, the optimal diameter of calcium particles enlarges. The usage of calcium decreases with increasing particle size at a given depth of molten steel and oxygen and sulfur contents. Calcium particles of less than 0. 002 m in diameter will be wholly utilized under the condition of the temperature of 1823 K, the mass fraction ofS of 0.012% and the depth of molten steel of 3 m. The conversion rate of calcium is 84.4% when the particle diameter is in the range of 0. 002 to 0.003 m.