The morphology of block copolymer and nanoparticle composites depends not only on the copolymer architecture but also on the surface energy, geometry, and volume fraction of the embedding nanoparticles. Toward a quantitative description of the composite structure and associated thermo-mechanical properties from a molecular perspective, we examined the performance of a nonlocal density functional theory that accounts for the excluded-volume effects and intra- and inter-chain correlations self-consistently. It is predicted that, within the lamellar structures of symmetric block copolymers, neutral particles are localized at the microdomain interface, leading to a reduction of the lamellar thickness. Conversely, particles that are energetically biased to a particular microdomain expand the block copolymer lamellar structure. The dilation or shrinkage of the lamellar thickness also depends on the particle packing density. Both particle dispersion and particle-polymer interfacial structure are highly sensitive to the ratio of the particle diameter to the lamellar thickness. While small nanoparticles may either increase or reduce the extensional moduli of the composite material depending on the nanoparticle volume fraction and polymer−particle interactions, large particles always enhance the mechanical properties regardless of the polymer−particle interactions. The theoretical predictions are found to be in qualitative agreement with simulation results and experiments.