Mixtures of diblock copolymers and nanoscopic spherical particles can yield well-ordered hybrid materials, which can be used for separation processes, catalysis, and optoelectronic applications. Predicting the morphologies of these systems is difficult because the final structures depend not only on the characteristics of the copolymer but also on the features of the particles, Combining self-consistent field and density functional theories, we develop a model that allows us to determine the equilibrium or metastable phases of diblock copolymer/spherical nanoparticle composites, without making a priori assumptions about the structure of the system. Using this model, we illustrate various examples where mixtures of diblocks and nanoparticles self-assemble into mesoscopically ordered phases. The model can be generalized to other types of copolymers and particles and can be modified to include homopolymers or solvent molecules. Thus, the technique constitutes a useful tool for determining the structures of a large class of nanocomposites.