The four-phase-model method is used to study residual stress field of composite ceramic with eutectic interphases. Considering matrix, interphase and fiber inclusion are isotropy, the residual stress distribution of composite ceramic with eutectic interphase is obtained. The size dependence of residual stresses is analyzed. The residual-stress-field in the eutectic ceramic rod relates to the stiffness, thermal expansion coefficients, shape and volume content of inclusions and interphases in the eutectic ceramic rod. On basis of the residual-stress-field in the eutectic ceramic rod, the frictional force on the boundary of the eutectic ceramic rod can be computed. Then the frictional work is given. The toughening mechanism of residual stresses is gotten. The result shows that the toughening value is dependent on the size of fiber inclusion in the eutectic rod.