A silicon nitride ceramic with unidirectionally aligned beta -Si3N4 elongated grains (UA-SN) was fabricated by sintering the extruded Si3N4 green body with a small amount of rodlike beta -Si3N4 seed. The effect of anisotropy in microstructure on tribological properties was investigated, compared with a fine-grained Si3N4 without seed. Block-on-ring tests without lubricant were conducted at sliding speeds of 0.15 and 1.5 m/s, with a normal load of 5 N and a sliding distance of 75 m, using the UA-SN and Si3N4 without seeds as block specimens and commercially supplied SO4 as ring specimens. For UA-SN, tribological properties were evaluated in three directions with respect to the grain alignment: the plane normal to the grain alignment, and in the direction parallel to or perpendicular to the grain alignment in the side plane. For both sliding speeds, the plane normal to the grain alignment exhibited the highest wear resistance, and the worn surface of this plane was quite smooth, in contrast to the other specimens whose surfaces were irregular owing to grain dropping. It is considered that the high wear resistance achieved in this plane is attributable to the inhibition of crack propagation along the sliding surface by the stacked elongated grains normal to the sliding surface.