The strength of layered composites consisting of beta-sialon and Si3N4 layers, which were prepared by hot pressing, was investigated. The strength increased as the thickness of the sialon (outer layer) decreased, and reached almost the same level of Si3N4 (inner layer) when the sialon thickness was 250-300 mu m. No specific fracture morphologies were recognized around the interface of sialon and Si3N4. The aluminium concentration changed sharply around the interface, while the yttrium tended to diffuse deeper than aluminium. This tendency was remarkable in the samples hot-pressed at higher temperature (1900 degrees C). The existence of compressive residual stress in the surface sialon layer was revealed and the residual stress increased as the sialon thickness decreased down to 250-300 mu m. The increase of strength with the decrease of sialon thickness was discussed based on the mechanical calculations in which the residual stress was considered. This calculation approximately agreed with the results of the samples hot-pressed at lower temperature (1800 degrees C). However, the strength of the samples hot-pressed at 1900 degrees C was much higher than the prediction in the thin range of the sialon thickness. The deep diffusion of yttrium into the sialon layers was thought to be one of the causes of this unpredictable effect.