The erosion resistance of α-Al2O3 ceramics in flowing suspensions of solid particles was investigated with a rotary disk erosion wear tester. The effect of different abrasive particle hardness on its erosion resistance was studied and the microcosmic failure mechanism of its surface was analyzed. The results show that the erosion resistance of α-Al2O3 is more and more excellent than that of the metal Cr15Mo3 with the hardness enhancement of the abrasive particles. When SiO2, Al2O3, SiC is in turn selected as the abrasive, the erosion resistance of α-Al2O3 ceramics is 4.06, 5.20, 5.77 times than that of Cr15Mo3 respectively due to the different hardness of such abrasives. Due to erosion, Cr15Mo3 is worn out most in the whole, the erosion holes are very clear and its failure pattern is in “W” shape. The wear of α-Al2O3 mainly occurs in the binding phase of the crystal boundary for its grain is fine and dense enough. So although the crystal grains are exposed at last, no breaks and cracks can be found on the erosion surface. Its failure pattern is in “U” shape.