Two kinds of sintering additives based on the polysiloxanes or polysilazanes filled with nano-sized powders as SiAlON precursors were tested for the densification of Si3N4-based ceramics. The results showed that both systems can be successfully used as additives for the preparation of Si3N4 ceramics with favorable mechanical characteristics. The ceramics were sintered with 18wt% of preceramic polymer-based mixture, and good fracture resistance and high hardness values were obtained after sintering in optimized conditions (temperature, dwell time, nitrogen pressure). Higher densification temperatures and longer holding times were required for sintering of samples with polysilazane-based precursors. The best toughness values were approximately 5MPam(0.5), while the highest hardness was about 19GPa. The differences in mechanical properties of the prepared composites can be related to the phase composition, microstructure and different chemical bonds present in the ceramic residue generated upon pyrolysis and final densification.