Abstract: The calcium treatment effect,the intermediate product formation and the modification routes of alumina inclusions by different intermediate products after calcium treatment were investigated by observing the shape and composition of these inclusions. It is found that blocky or rectangular solid inclusions finally change into spherical liquid inclusions,but their modification degrees are different. According to thermodynamic analysis,the higher the Als and S concentration in molten steel,the more difficult the modification of alumina inclusions is. Ca S and CaO exist as intermediate products in a short time after calcium treatment. A kinetic model of intermediate product formation shows that the critical[S] mass fraction of the formation of different intermediate products is 11.1×10^-6 with 4×10^-6 of dissolved oxygen. The modification routes of alumina inclusions by different intermediate products can be derived from the above model and experimental results.