This paper deals with densification and a®b phase transformation of Si3N4, with a constant b-Si3N4 seeds concentration regarding sintering temperature and different additive types. The seeds of b-Si3N4 were obtained by sintering of silicon nitride in a-form and additive mixture consisting of Y2O3 and SiO2. Two different Si3N4/additive mixtures (Y2O3 + Al2O3 and CeO2) containing constant amount of seeds were prepared and tested. Characterization of sintered samples involved phase analysis by X−ray diffraction and density measurements. The results indicated that in the presence of yttria-alumina mixture both the phase transformation and the densification were enhanced as compared to samples containing only CeO2 as a sintering additive. The reasons for observed behavior are discussed in detail.