Surface modification by ion implantation has been conducted to improve the tribological properties of a high-strength and high-fracture-toughness unidirectionally aligned silicon nitride (UA-SN). B+, N+, Si+, and Ti+ ions were implanted into the planes parallel and normal to the grain alignment of the UA-SN with a fluence of 2 x 10(17) ions/cm(2) at an energy of 200 keV. The ion implanted UA-SN showed a dramatic improvement in wear resistance. For example, the specific wear rate of the Si+-implanted specimen in the direction parallel to the grain alignment was reduced to a value of 3 x 10(-10) mm(2)/N, equal to 1/20 of the unimplanted one. Cross-sectional transmission electron microscopy indicates the high wear resistance was attributed to the amorphous surface caused by the ion implantation.