Abstract: Eutectic composite ceramic containing lamellar inclusion possessed good strengths at room temperature and high temperature. Firstly, under the conditions that the matrix, interphase and the lamellar inclusions are isotropic, the size dependent micromechanical stress field of the composites can be achieved. Then, stress concentration generated by the dislocation pile-up in interphase was analyzed and the maximum stress in the matrix was obtained. When the maximum stress is equal to the breaking strength of the matrix in molecular theory, the ultimate shear stress of the composites can be derived. Finally, the analytic expression of the eutectic composite ceramics can be obtained based on the relationship between shear stress and the load. The result shows that the fracture strength of such composite ceramics had obvious size dependence.