Abstract: A constitutive model of particulate reinforced composites (PRCs) which can describe the evolution of debonding damage, matrix plasticity and particle size effects on deformation and damage was developed. An incremental damage model of PRC based on Mori-Tanaka's mean field concept had been extended to three-phase composites for interpreting particle size effect. The interphase was perfectly incorporated into the present micromechanics model as a third phase with the help of double-inclusion model. Progressive damage was controlled by a critical energy criterion for particle-matrix interfacial separation. Based on the developed model, the influences of progressive debonding damage, particle size and interphase properties on the overall stress-strain response of PRC were discussed. Finally, the particle size effect on the mechanical behaviors of composites was clearly interpreted from the role of the interphase, which was different from all the existing researches.