The refining slag plays an important role in controlling the cleanliness of the steel during refining process. The effect of the slag on the composition modification, dissolution, and adsorption into the slag of inclusions in the steel are discussed in this chapter. The influence of CaO/Al2O3 ratio and the basicity of slags on the cleanliness of both Si-Mn-killed and Al-killed steel was introduced. The Al2O3 inclusions in the 304 stainless steels rose up with the increase of slag basicity and the Al2O3 content in the final slag. The application of low basicity slag and low Al2O3 content in the slag was a key factor to control of the composition of inclusions. A thermodynamic model was developed to investigate the effect of slag composition on the cleanliness of the 304 stainless steel. During the production process of the bearing steel, with the basicity of the slag reducing from 8 to 5–6, the CaO content of inclusions in the final steel product reduced from 30% to less than 5%. The formation of CaO-Al2O3 and CaO-Al2O3-MgO inclusions was significantly inhibited.