Abstract: Although much efforts have been made to understand the relationship between microstructures and deformation behaviors of particle-reinforced metal matrix composites (PRMMCs) during the past several decades, there are still some phenomena remained to be understood, one of which is size effects in PRMMCs. Recently, many experimental results demonstrate that reinforcing particle sizes have a significant influence on yield and flow stresses of PRMMCs. However, various micromechanical models which are based unpon homogenization technique for particulate composites predict particle volume fraction effects on deformation behavior of composite materials but show an independence of particle sizes. According to this observation, a strengthening-strain gradient relation for particle-reinforced metal matrix composites was developed by madding use of the concept of geometrically necessary dislocation and proposed dislocation model. A comparison with existing experimental results demonstrates that the relationpredicts a dependence of yield and flow stresses of composites on reinforcing particle sizes. From this relation, we found that the strengthening effect of mechanical behavior of composites is controlled by both characteristic microstructure geometrical parameters and strain gradient in matrix. For a given PRMMCs, the strengthening effect of the composite is completely controlled by particle size or strain gradient in the matrix. This tells us that the strain gradient in matrix may be an important factor controlling deformation and fracture behavior of heterogeneous material systems. Moreover, effects of characteristic microstructure geometrical parameters on deformation localization of PRMMCs are also discussed.