Silicon nitride based ceramics have attracted considerable attention as good candidates for structural applications due to their excellent mechanical properties including strength, hardness, fracture toughness, and high temperature strength. These properties are strongly influenced by grain size and morphology, and by the degree of crystallinity and chemistry of grain boundary phases. In this work, the microstructure of Si3N4 densified with Nd2O3, Y2O3 and Al2O3 sintering additives was studied. Sintered samples were polished and plasma etched for microstructural analysis using scanning electron microscope. Quantitative evaluation of materials microstructure was accomplished using Quantikov software. Fracture toughness was measured by Vickers indentation method. The observed microstructure is typical of silicon nitride based materials and is characterized by high aspect ratio -Si3N4 grains embedded into a matrix formed by sintering additives. The grain size distribution was shown to depend on the alumina content. Fracture toughness increase with the increase of average grain diameter.