A tribofilm was formed during wear tests of a Si3N4-gray iron pair lubricated with distilled water. In order to clarify the formation process Of the film, wear tests of fhe Si3N4-gray iton pair with different sliding distances were carried out on a ring-block tester using distilled water as iubricant. After the tests, the worn surfaces of the gray iron specimens were examined under SEM. Furthermore, the components and the structure of the film were analyzed by AES, XPS, FTIR and XRD. Combining the obtained results, the following aspects are obtained. During the wear tests of a Si3N4-gray iron pair lubricated with distilled water, oxidation and hydrolysis of Si3N4 occur on the wearing surfaces, and the reaction product may be adsorbed by graphite flakes in the iron. As a result, a tribochemical film with a certain thickness and area, which consists of silicagel and graphite, is formed on the wearing surface. The film protects both Si3N4 and gray iron and makes the paired surfaces smooth. Therefore. the friction coefficient of the pair iS lowered down to as low as 0.002, and the total wear rates of Si3Na and Iron are near zero. However, because the products of oxidation and hydrolysis of Si3N4 cannot be accumulated on the wearing surface of carbon steel to form an eflfective tribofilm, both the friction coeffcient and the wear rate of the Si3N4-carbon steel pair lubricated with distilled water are high.