Microwave pretreatment is an effective method to promote grindability of coal samples along with moisture reduction. A contrast test was designed to explore a mechanism of grindability improvement process of coal by microwave pretreatment. Coal samples of 2–1 mm were investigated. The moisture content, size distribution and density composition of grinding products of dry coal with thermal drying (DTD), dry coal with microwave pretreatment (DMP) and wet coal with microwave pretreatment (WMP) were measured. A scanning electron microscope analysis showed that DMP had more micron-sized cracks than WMP, meanwhile, WMP had more cracks than DTD after microwave pretreatment and thermal drying. The results of grinding tests showed that yield of -0.125mm size fraction increased with the decrease of moisture content, and was equal to 43.11, 45.03 and 47.09% of DTD, WMP and DMP, respectively, after 15 minutes grinding for the same moisture content. The results showed that cracks caused by dehydration and selective heating characteristics were the main factors responsible for improvement of grindability, however, thermal evaporation of inherent moisture within the coal structure had almost no effect on grindability of coal.