A study has been made of the chemical composition and mechanical properties of Ti+-implanted Si3N4 surface layers. Implantation of 48Ti+ was performed with doses ranging from 10 to 1017 ions cm”2 at an energy of 150 keV, and at nearly room temperature. XPS was used to analyze the depth dependence of atomic fraction and chemical bonding states of Ti+-implanted layers. The near-surface hardness was measured by a Vickers hardness tester. The friction and wear properties were measured under unlubricated conditions at room temperature using a pin on disk-plane and a block on wheel-periphery configurations, in which the pin and wheel used were AISI1045 and ASTM Wl-9, respectively. Implanted Ti-atoms formed a gaussian distribution predicted by the range theory. At the average projected range, most of Ti-atoms existed as a metallic state and TiN bonding was also formed. Oxygen and carbon were found near the surface layers. In addition to the surface peak, O-atoms accumulated in front of the average projected range of Ti. Such O-atoms formed bonds of Si-oxides and Ti-oxides. Carbon existed as a graphitic state. With increasing a Ti dose, the near-surface hardness decreased, and the wear rate increased at the running-in stage having the high friction coefficient. The steady wear attributed to the stable friction coefficient appeared after the running-in stage, although such a stable stage was not observed for unimplanted Si3N4. The mechanism for the change in mechanical properties of Si3N4 induced by Ti+-implantation will be discussed in relation to XPS characteristics.