Abstract: The desulfurization of CaO-Al2O3 particles in the molten steel was observed in situ through the high temperature confocal scanning laser microscopy. The effect of aluminum and silicon contents in the molten steel on the desulfurization was analyzed. With the T.Al content in the steel increased from 0 ppm to 1100 ppm, the CaS content in the CaO-Al2O3 particle increased from 2.1wt% to 84.84wt% after the reaction for 90 s. With the silicon content in the steel increased from 0.6wt% to 3.0wt%, the CaS content in the CaO-Al2O3 particle increased from 1.53wt% to 79.01wt% after the reaction for 90 s. It was indicated that the increase of aluminum and silicon contents in the steel, promoted the desulfurization of CaO-Al2O3 particles. A kinetic model was established to predict the CaO-Al2O3 particle composition, and the diffusion coefficient of sulfur in CaO-Al2O3 particles was calculated as 9.375×10-10 m2·s-1 at 1600°C, which provided a new method for the calculation of diffusion coefficient.