The effects of microwave irradiation on the crystallization of 40%CaO-40%SiO2-20%Al2O3 synthesized slag and actual blast furnace slag were investigated using a 28 GHz multimode microwave irradiation system, a commercial microwave oven (2.45 GHz), and an electric resistance furnace. While it was observed that the 2.45 GHz centimeter-wave had little effect on the samples, the 28 GHz millimeter-wave was found to accelerate crystallization significantly, especially at lower temperatures at which the crystalline phases are never generated by external heating. Further, the effects of the precipitated phases and impurities on the crystallization with the 28 GHz microwave irradiation were investigated. The temperature of the glassy slag increased more rapidly when compared with the crystalline phases that exist in the actual blast furnace slag. In addition, a small amount of impurity was found to improve the heating property of the slags. © 2007 ISIJ.