Abstract: Experimental study has shown that the interfacial bonding between fibers and matrix can be significantly improved by introducing nanophases into fiber reinforced polymer composites. The reinforcing effects of various nanophases on the obtained multi-scale composites were found different, which were associated with the shape and dimension of the nanophases. In this paper, a multiscale model was proposed based on the cohesive energy model to explore the source of the difference in reinforcing efficiency introduced by the three kinds of typical nanophases, i.e., carbon nanotubes, spherical fullerene nanoparticles and graphene nanoplates. The model elucidates how the shape and quantity of nanophases influence the strength of interfacial bonding in multiscale composites. The proposed model was verified according to the experimental results obtained through transverse tension tests of fiber bundle composites. The theoretical prediction shows a good agreement with the measured data.