The process of densification and development of the microstructure of primary mullite-ZrO2/Y2O3 powders by gradually adding α-β Si3N4 nanopowder from 1 to 5 wt.% by traditional and spark plasma sintering, were investigated by means of DTA, XRD, SEM, as well as from the point of view of characteristic ceramic and mechanical properties. The main processes of DTA for all samples are characterized by a low-pitched endo-effect, when gradual mullite formation and noticeable densification when reaching the temperature up to 1200 – 1400 °C is started. It is characterized by shrinkage and density both for traditionally and by SPS sintered samples. The influence of Si3N4 additive for density characteristics is insignificant for both sintering cases. For SPS samples, the density value reaches up to 3.33 g/cm3, while for traditionally sintered – 2.55 g/cm3, compressive strength for SPS – grows with Si3N4 additive and reaches 600 N/mm2, in case of traditional sintering it drops down to approximately 200 N/mm2. The basic microstructure of ceramic samples sintered in a traditional way and by SPS basic is created from mullite (or pseudomullite) crystalline formations with incorporation of ZrO2 grains. The microstructure of ceramic samples that were sintered by SPS shows dense, mullite crystals which are very densely arranged and which do not have the characteristic prismatic-shape. Traditional sintering process causes creation of voids in microstructure, which with an increasing amount of Si3N4 additive are filled with mullite crystalline formations.