<p>Many experiments have shown that micro-particle reinforced metal matrix composites (MPMMCs) display a strong particle size effect on mechanical behavior. Meanwhile, the stress concentration near the particle phase leads to matrix damage and interface debonding for composites in service. In this research, a modified conventional theory of mechanism-based strain gradient plasticity (CMSG) considering the damage effect, and a cohesive zone model are used to predict the mechanical behaviors of MPMMCs. The particle size effect and matrix damage behavior are characterized by modified CMSG while the interface debonding is controlled by the cohesive zone model. Details about the local distributions of strain, strain gradient and stress fields have been captured. An interesting phenomenon is found that matrix damage enhances the strain and strain gradient of the matrix, but interface debonding does the opposite. Both the interface debonding and matrix damage weakened the strength of composites. As a result, the numerical predictions agree well with both uniaxial tension and compression experiments. Furthermore, this work finds interface debonding takes the dominant role of damage mechanisms in uniaxial tension cases. However, matrix damage is dominated in compression cases. The present research should provide a comprehensive understanding of the mechanical behaviors of MPMMCs in service, which is also helpful for optimal designs of such advanced composites.</p>