In this study, porous Si3N4 ceramics were fabricated by carbothermal reduction reaction between silicon dioxide and carbon. The influences of different starting powders and sintering additives on microstructure and mechanical properties were investigated. XRD analysis demonstrated the formation of single-phase β-Si3N4 except for glass phase and minor of α-Si3N4 phase. SEM analysis showed that the resultant porous Si3N4 ceramics occupied fine microstructure and uniform pore structure. The samples with fine starting powder showed fine, high aspect ratio of β-Si3N4 grains and good mechanical properties. The addition of Al2O3 accelerated the densification of porous Si3N4 ceramics. With an increasing in the sintering additive content, the porosity decreased, the flexural strength increased.