To investigate the influence of crystal structure and solid solutions of certain elements in Si3N4 on aqueous tribological properties, seven specimens were fabricated. Solid solutions of aluminum and oxygen made the friction coefficient and wear increase in both alpha -type and beta -type crystal structures. In particular, alpha -SiAlON, in which yttrium is also incorporated into the alpha -Si3N4 lattice, showed a higher friction coefficient and wear than other specimens containing the same concentration of Al or O.