For Ceramic particle Ni base alloy composite, experiments and damage evolution finite element analysis shows that arc-microcracks happen only at the interfaces between the particles and matrix, and then extend to the matrix. Composite strength is coincident with matrix failure. First, the three-phases model is used to determine the external strains of two-phases element. Then micromechanical stress field in the matrix is obtained. Based on the generalized thermodynamic force in damage process, the damage equivalent stress can be computed. As the damage equivalent stress is equal to the ultimate stress of the matrix, the failure stress of matrix in composites is given.