Abstract: The blast furnace（BF） slag bearing high TiO₂ was processed under microwave irradiation in order to induce micro-cracks in it and to improve the grindability of the slag.During inducing crack in the slag,it was found that the slag could be effectively heated by microwave,and thermal runaway was occurred.In order to investigate the mechanism for effectively heating of the slag and the reason of occurring thermal runaway in the slag irradiated under microwave field,different kind of synthesized slag was heated in microwave field.Experimental results showed that the existence of CaTiO₃ played a key role in heating of the slag under microwave irradiation.The permittivity of CaTiO₃ was measured with Network Analyzer.It was determined that the permittivity of CaTiO₃ was higher than general material,which quantitatively explained the role of CaTiO₃ on heating of the BF slag bearing high TiO₂ irradiated under microwave field.Furthermore,the permittivity of CaTiO₃ increased with the temperature rising,which resulted in the heating way of positive feedback.The heating way of positive feedback was one of the main reasons for the occurrence of thermal runaway in the BF slag bearing high TiO₂.