Several silicon nitride ceramics with different compositions and amounts of sintering additive were fabricated by hot-press method. The effect of sintering additive on the sinterability, flexural strength and fractography was examined. The flexural strength of silicon nitrides sintered with alumina as an additive were not high at approximately 400MPa, although the sinterability was good in that bulk densities of 3.05-3.18 × 103kg m-3 were obtained. The sinterability of silicon nitrides sintered with yttria as an additive resulted in low bulk densities of approximately 2.7 × 103kg m-3, with flexural strength ranging from 370 to 650MPa. The sinterability of silicon nitrides that were sintered with both yttria and alumina additives resulted in relatively high bulk densities of approximately 3.21 × 103kg m-3 and extremely high flexural strength in the range of 900 to 1140MPa. The fractography of fracture origin was dependent on the type of sintering additive used; an inclusion size of approximately 30μm for the alumina additive, pores for the yttria additive and small secondary sintering defects for both yttria and alumina as additives were observed at the respective fracture origins.