Sintering additives Y2O3 and Al2O3 with different ratios ((Y2O3/Al2O3) from 1 to 4) were used to sinter Si3N4 to high density and to induce microstructural changes suitable for raising mechanical properties of the resultant ceramics. The sintered Si3N4 ceramics have bimodal microstructures with elongated Si3N4 grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. Pores were found within the grain boundary phase at the junction regions of Si3N4 grains. The hi-hest average aspect ratio (length/width of the grains) of similar to 4.92 was found for Y2O3/Al2O3 ratio of 2.33 with fracture toughness and strength values of similar to 7 MPam(1/2) and 800 MPa, respectively. The effect or microstructure, specifically grain morphology, on mechanical properties of sintered Si3N4 were investigated and found that the aspect ratio of the elongated grains is the most important microstructural feature which controls mechanical properties of these ceramics.