It is now Well known that the level of strengthening and toughening in silicon nitride ceramics depends to a. great extent on the amount and type of additives used to sinter it. Alumina and yttria are by far the most successful additives in producing strong and tough Si3N4 ceramic. Their role is to provide diffusion path for atoms and to help induce growth of elongated beta-Si3N4 in the matrix of equiaxed or slightly elongated grains. These elongated grains can acts as reinforcemen through the crack bridging and crack deflection mechanisms. Most recently, it has been shown that the crack bridging is one of the most powerful mechanisms of toughening and, theoretically, is capable of giving toughness as high as 30-40 MPa.m(1/2). In this mechanism, the toughening is controlled by the elongated grain aspect ratio, the interfacial strength and the volume fraction of the elongated grains. In this paper, systematic studies of the effect of yttria to alumina ratio on strength and toughness of pressureless sintered silicon nitride ceramics were presented. It was found that both fracture strength and toughness are strongly affected by the ratio Y2O3/Al2O3. The highest density was achieved with samples having Y2O3/Al2O3 ratio of 5/5.5, whereas the highest toughness and strength were achieved with samples having the yttria to alumina ratio of 7/3.5.