The resistance to cavitation erosion of one Yb-α-Sialon ceramic and two Dy-α-Sialon ceramics (Dy-α-Sialon-1 and Dy-α-Sialon-2) was investigated in deionized water using an ultrasonic vibratory apparatus. The incubation periods of Dy-α-Sialon-1, Dy-α-Sialon-2 and Yb-α-Sialon ceramic were 210 min, 300 min and 360 min, respectively. Surface roughness and area fraction of cavities were found good indicators for the evolution of ceramic serosion. The microstructural features, including main phase(α phase), inhomogeneities of the microstructures and type of rare earth oxide, were correlated to the cavitation erosion behavior. It was found that the elongated α-Sialon grains in Yb-α-Sialon and Dy-α-Sialon-2 produced a self-toughening effect to increase the incubation period. The aggregation of Dy₂O₃ in Dy-α-Sialon-1 had an adverse effect on the resistance to cavitation erosion. Impurity phases and type of rare earth oxide had impacts on resistance to cavitation erosion. The findings in this work indicate that the cavitation erosion resistance of α-Sialon ceramics could be improved by microstructure optimization.