Abstract: The aim of this paper is to study the effect of the interface on the macroscopic strength of ductile composites on the microscopic scale. An interface model based on microscopic fluctuation displacement (MFD) is proposed. Considering that the debonding of an interface can be regarded as a kind of material fracture, the strength of the interface is characterized by means of fracture toughness. The numerical results show that the effect of the interface on the off-axial strength of fiber-reinforced composites is most intensive and that the interface failure changes the distributing of the plastic zone in the matrix.