To study the effect of process-related oxidation on microstructure and densification of β′-o′ sialon, green compacts of a silicon nitride powder and boehmite sol were prepared. With progressive oxidation of the silicon nitride powder, the amount of o′-phase increased and grain size of o′-phase was changed. The different densification was caused by o′-phase content. The experimental result showed that, the transition liquid phase in the specimen of low oxygen contents formed at higher tempeature than that of high oxygen contents.