Block copolymer nanocomposites are obtained when the nano-particles are well dispersed in the matrix of microphase-separated block copolymers. In this work, we use 3D real-space self-consistent field calculations with high accuracy to study such nanocomposites. As the first step, our model consists of a single particle in the lamellae of symmetric diblock copolymers with periodic boundary conditions applied in all directions. We study how the particle loading (volume fraction), shape and interactions with the two blocks affect the location and orientation of the particle as well as the lamellar period. The equilibrium morphology of the system at a given set of parameters is identified through free-energy comparison. Such structural information serves as the basis for further investigation on the mechanical properties of diblock copolymer nanocomposites. Our results are compared with available theoretical and simulation studies in the literature.