The large aspect ratio of interfacial area per unit volume at CNT/matrix interfaces may significantly influence the macroscopic behavior of CNT-reinforced composites. The property of interfaces is governed by the cohesive law, which is determined from both van der Waals forces and chemical covalent bonds. This nonlinear cohesive law is incorporated in the micromechanics model in the current work to study the mechanical behavior of CNT-reinforced composites. It is found that carbon nanotubes can improve the macroscopic behavior of the composite at small strain, but tend to weaken the composite at relatively large strain because of the interface softening behavior or debonding. The increase of interface adhesion caused by the creation of chemical covalent bonds may significantly improve the composite behavior at large strain.