Silicon nitride with unidirectionally oriented elongated grains was fabricated by the combination of seeding and extrusion process. Development of such highly anisotropic microstructure is due to the high shear stress exerted during the extrusion process, which allowed the unidirectional alignment of rod-like seed particles. Small matrix grains were almost consumed by selective grain growth into larger grains grown from seed particles, consequently, large elongated grains with high aspect ratio were dominant in the microstructure of this material. Such microstructure development is related to the fact that the unidirectional alignment of seeds along a precise direction inhibits mutual impingement of elongated grains during growth along their length direction. Both high strength of about 1.4GPa and high fracture toughness of about 14MPa·m1/2 could be achieved by testing in the direction parallel to grain alignment.