The phase behaviors of diblock copolymers confined in thin films with two identical preferential surfaces are investigated using the self-consistent field theory. Around 20 morphologies, including centrosymmetric and non-centrosymmetric ones, are considered to construct the two-dimensional phase diagram with respect to the volume fraction and the film thickness, while the interaction parameter chi N and the surface preferences are fixed. When these morphologies are classified into four categories of ordered phases-sphere, cylinder, perforated lamella (corresponding to gyroid phase in bulk), and lamella-the phase diagram directly reveals the impact of the film confinement on the order-order transitions as a function of volume fraction via the comparisons to those in bulk. Our results also provide a comprehensive understanding over the dependence of the structure formations on the film thickness for each volume fraction.