Densification as well as the alpha --> beta phase transformation of Si3N4 were monitored as a function of activation time of the BaCO3-Al2O3-SiO2 additive mixture. The composition of the ternary mixture corresponded to celsian (BaAl2Si2O8-BAS). Previously, mechanically activated powder mixtures for various lengths of time were added to Si3N4 in the amount of 10-30%. Sintering was performed at 1650-1700 degreesC in nitrogen atmosphere up to 8 h. The changes in densification degree, as well as phase composition, were followed as a function of heating time and the time of mechanical activation of the additive mixture. The results obtained showed that mechanical activation retarded densification in samples heated up to 1700 degreesC. On the other hand, for the constant sintering time, alpha --> beta transformation of Si3N4, was enhanced with increasing activation time, and the amount of additives.