The relations among the compositions of inclusion, steel and top slag were thermodynamically studied at 1823 K in tire cord, valve spring, ultra low carbon sheet, bearing and sulfur free-machining steels. In case of a tire cord and valve spring steels, the equilibrium among inclusion, steel and top slag was found to be established in practice with respect to Si and Mn. The contents of Si, O and Ca in an ultra low carbon sheet steel were determined as a functin of top slag composition, and the higher oxygen contents observed in practice were explained by the effect of Fe_tO in slag. Themodynamic discussion was made for the formation of spinel inclusions in a bearing steel and the precipitation of CaS in a sulfur free-machining steel.