The densified α-sialon ceramics with the compositions RE0.333Si10Al2ON15 (RE= Yb,Y, Dy, Sm and Nd) were prepared by a two-step hot-pressing sintering. The ceramics doped with smaller cations (Yb3+, Y3+ and Dy3+) are fully composed of α-sialon, while the larger cation-doped ceramics (Sm3+ and Nd3+) exist a few M′ (RE2Si3-xAlxO3+xN4-x) phases. A small amount of β-sialon phases are also found in the Nd-doped sialon. Microstructure observation indicates that Yb-α-sialon consists of equiaxial grains, but when increasing the radius of the doped cations, the elongated α-sialon grains form and the aspect ratio of grain increases slightly. TEM observation indicated that almost no intergranular phases exist in the α-sialon doped with smaller cations, but a few exist in Sm- and Nd-doped α-sialon. Some elongated α-sialon grains with grain core were found. EDX results suggest that the compositions of the grain shell and that of the core are different. Surrounding the grain core, some misfit dislocations were seen.