Abstract: A systematic investigation was carried on the variations of total oxygen content and inclusion types in 39Mn2V steel before and after calcium treatment.The modification mechanism of Al₂O₃ and MnS was analyzed by thermodynamics.The results show that Al₂O₃ can be wholly modified into 12CaO·7Al₂O₃ and 3CaO·Al₂O₃ inclusions which are liquid at steelmaking temperature,while MnS just can be modified partially into CaS under the present amount of Ca-Si wire fed.