With the rapid development of economy, there has been an increasing number of cases where extremely hard rock is encountered. Thus, traditional mechanical excavation methods can hardly guarantee their efficiency and economy. Microwave-assisted rock breaking is a promising technique. Rock is formed by different minerals which have different microwave sensitivity related to their species, crystal forms and iron content. When the rock is exposed to microwave irradiation, differential expansion between minerals of different sensitivity causes thermal stresses inside and along the boundaries of the minerals, thus effectively weakening the rock. Two main factors affecting the weakening effect are microwave parameters and rock properties. Basically, rock strength declines as microwave power and irradiation time increase; a high power and short irradiation can cause a better weakening effect when consuming the same energy. The weakening effect of microwave on rocks can also be enhanced by a certain content of microwave sensitive minerals and a coarser particle size. The physical and chemical properties of minerals are sometimes greatly affected by microwave heating, which in turn affects the heating characteristics and weakening effect of the rock. Surface irradiation tests reveal that microwave can induce radial pattern cracks on the irradiation zone of large-size rock sample, thereby reducing the rock strength and improving the rock breaking efficiency.