Based on the previous work on Si3N4-ZrB2 [Wu et al. J Eur Ceram Soc;2017,37:4217], the influence of ZrB2 addition on the phase and microstructure evolution of Si3N4-ZrB2 composites was emphatically investigated, and the mechanical properties were compared with pure Si3N4 ceramics. It was revealed that the ratio of - to (+)-Si3N4 significantly increased from 14.3% in pure Si3N4 ceramics to 39.8% in Si3N4 with 15 vol% ZrB2 addition, indicating that the introduction of ZrB2 promoted - to -Si3N4 phase transformation. As a consequence, the microstructure of the composite showed the bimodal distribution, containing both elongated and equiaxed Si3N4 grains. For the pure Si3N4, Vickers hardness, fracture toughness and flexural strength was 22.8 GPa, 7.6 MPa m(1/2), and 334.5 MPa, respectively. In contrast, the composite of Si3N4-30 vol% ZrB2 simultaneously possessed an excellent combination of mechanical properties: 19.5 GPa in hardness, 9.8 MPa m(1/2) in toughness and 702.0 MPa in strength. Present study suggested that Si3N4-based ceramics with high hardness, high toughness, and high strength could be obtained by the combination of appropriate ZrB2 content and low hot-pressing temperature.