In the current chapter, a parameter of the difference between the original concentration and the saturation concentration of inclusions in the refining slag divided by slag viscosity (ΔC/η) is discussed, being used in relevant literatures. Disadvantages of the parameter are discussed, including its dimensional feature and the ignorance of the size of inclusions. A new concept of inclusion capacity of the slag was proposed to express the dissolution ability of inclusions in the slag. The dissolution capacity of inclusions in the slag was characterized using a dimensionless number Zh with definition as \( \mathrm{Zh}=g\cdot {\rho}_{\mathrm{slag}}^2\cdot \left({C}_{\mathrm{saturation}}-C\right)\cdot {d}_{\mathrm{p},0}^3{\eta}_{\mathrm{slag}}^{-2} \) , where C is the concentration of inclusions in the slag in mass%, Csaturation is the saturation concentration of inclusions in the slag in mass%, dp,0 is the initial diameter of the inclusion in m, ρslag is the density of the slag in kg/m3, ηslag is the dynamic viscosity of the slag in kg/(m⋅s), and g is the gravitational acceleration rate in m/s2. The inclusion capacity of the slag, the Zh number, can be used to predict the dissolution rate and dissolution time of inclusions in the refining slag, to calculate the rate constant of the dissolution reaction and the diffusion coefficient of inclusions in the slag, and to calculate the removal time of inclusions from the molten steel.