In discontinuously-reinforced composites, their mechanical properties are affected by cracking or debonding damage and size of reinforcements such as particles or short-fibers. This presentation deals with a constitutive model of discontinuously-reinforced composites which can describe the matrix plasticity, evolution of cracking or debonding damage of reinforcements, and reinforcement size effects on deformation and damage. The model is developed based on the Eshelby equivalent inclusion method and Mori-Tanaka mean field concept, and can be applied to particulate- or short-fiber-reinforced composites with progressive cracking or debonding damage. Influence of progressive damage and size of reinforcements on the stress-strain relations of the composites are demonstrated based on the numerical results by the model. Furthermore, a finite element method (FEM) is developed based on the model for particulate-reinforced composites with progressive debonding damage. As an example, FEM analysis is carried out on a crack-tip field in the composites, and the influence of particle size and debonding damage on an elastic-plastic singular field around a crack-tip is discussed based on the numerical results.