In order to examine the fundamental boundary lubrication properties of ceramics, reciprocating friction experiments of silicon based ceramics such as silicon carbide and silicon nitride were conducted with trialkyl phosphites and other oil additives. When ceramics were slid against ceramics, trialkyl phosphites with long carbon chains reduced the friction of silicon nitride markedly; the friction coefficients decreased with an increase in the carbon chain length. Other oil additives, however, did not greatly affect the friction. When ceramics were slid against metals, additives containing chlorine or sulfur increased friction of certain sliding couples. On the other hand, a trialkyl phosphite reduced friction and the friction coefficients increased with an increase in the maximum Hertzian contact pressure.