Changes of density, the alpha-beta phase transformation, and composition of grains and grain boundaries during sintering of Si3N4 with various sintering conditions using additives of Y2O3 and Al2O3 were investigated. The phase determination of individual Si3N4 grains was performed by convergent beam electron diffraction. The relations between densification and transformation were divided into two groups, depending on the additive compositions. Aluminium dissolution into Si3N4 grains occurred mostly during alpha-beta transformation process. The concentrations of aluminium and oxygen in the grain boundaries decreased as the alpha-beta transformation progressed.