The aim of this study is to produce highly dense Si3N4 based composites with good mechanical properties and bioactivity. Si3N4 ceramics without using sintering aids, Si3N4-HA and Si3N4-HAGNP based composites have been produced by spark plasma sintering (SPS) at temperatures of 1525-1550 degrees C. The effect of reinforcement type and content on the densification behavior, phase analysis, microstructural development, mechanical properties, and in-vitro bioactivity behavior of Si3N4 were systematically investigated. Monolithic Si3N4 that contains a high amount of (3-Si3N4 phase (similar to 87 wt%) was produced by nearly full densification (similar to 99%). Hydroxyapatite (HA) was used as a starting powder during the preparation of binary and triple composites to provide bioactivity to Si3N4, and after sintering, HA transformed into tricalcium phosphate (beta-TCP and alpha-TCP) polymorphs. The incorporation of GNPs had a positive effect on the stability of beta-TCP phases at higher sintering temperatures. The improvement in indentation fracture toughness of the samples with GNP reinforcement was mainly attributable to pull-out and crack deflection mechanisms. In-vitro bioactivity of GNP added composites enhanced with increasing alpha-TCP content. More calcium phosphate-based particle formation was observed in Si3N4-HA-GNP composites compared to the Si3N4-HA.