Abstract: The mechanical behavior of imperfect particle/matrix interface in particle-reinforced composites (PRCs) are defined by the cohesive zone model (CZM). Using mesoscopic mechanical Mori-Tanaka (M-T) method and dilute solution method, we studied the effect of imperfect interface stiffness on the effective modulus of composites. Results present that the monotonic increasing relation curves exist between the effective modulus and the particle/matrix imperfect interface stiffness for the composite with a certain volume fraction.The relation curves between the effective modulus and the imperfect interface stiffness with different volume fractions accounting for a certain composites converge to a unique critical point (CP), and the critical interfacial stiffness characterized by CP dominates the way of how volume fraction of particle affects the effective modulus. The effects of mechanical property of matrix and reinforcement phase, reinforced particle size on the critical interfacial stiffness of CP were studied. With effective modulus-interfacial stiffness relation curves and the experimental effective modulus, the estimated method of the imperfect interfacial stiffness for PRCs was proposed. The macro effective modulus of the composites was then predicted.