Abstract: The tribological characteristics of silicon nitride ceramics lubricated by water-containing lubricating oil were studied in this paper. The author indicated that water reacted chemically with substances on the friction surface and form hydroxycompounds, which were loosely bound with silicon nitride matrix, and were worn off easily under the action of higher pressure. In addition, the adsorption of water also decrease the lubricating ability of the oil, therefore, the more water content is the higher friction coefficient and wear of the ceramics.