The sintering behavior of mixed α /β -sialon ceramics, fabricated from α-Si 3 N 4 , AlN and Ln 2 O 3 (Y 2 O 3 , Er 2 O 3 and Yb 2 O 3 ), was examined by high-temperature dilatometry, and the densification process was discussed through the consideration of chemical reactions to form α - and β -sialon and kinetical analysis of shrinkage. The formation of α -sialon and melilite caused temporary retardation of the densification rate, while during the formation of β -sialon the densification proceeded remarkably. The densification accompanied by precipitation of β -sialon is ascribed to dissolution of residual α-Si 3 N 4 , α -sialon with low solid solubility and melilite into a liquid, which may result in an increase of liquid volume and in a decrease of liquid viscosity. These phenomena are considered to induce the solution-precipitation process, particle disintegration process and cooperative flow process of the particle-liquid mixture, yielding to rapid shrinkage. The densification at the initial and middle stages and the reactions from α -sialon and α-Si 3 N 4 to β -sialon accelerated in the order of Y 2 O 3 > Er 2 O 3 > Yb 2 O 3 addition, because the liquidus temperature and viscosity of liquid formed decreased in the order of Y 2 O 3 < Er 2 O 3 < Yb 2 O 3 addition.