The size effect of decohesion on the particle-matrix interface are studied for the particle-reinforced metal matrix composite via finite element method with strain gradient plasticity and cohesive interface failure. The surrounding matrix is modeled by the low-order finite elements with plastic strain gradient and Taylor dislocation. Assuming that the failure is caused by the interface decohesion only, the flow stress of SiC<SUB>p</SUB>/Al 356-T6 composite with the particle size of 7.5 μm and the volume fraction of 15% was fitted to Lloyd’ experimental data to get the reference fracture energy and the maximum traction. Based on them, a parametric study was performed to examine the size effect on the decohesion failure. The flow stress of composites showed distinct size effect of particle, cohesive energy, maximum traction and effective separation.