Phase coherence behavior in thin films of a diblock copolymer that exhibits microphase separation upon heating in the bulk has been analyzed through a proper mean-field free energy functional based on a compressible random-phase approximation theory. Phase coherent profiles at equilibrium and their decay lengths were obtained analytically for those copolymer films. Taking deuterated polystyrene-b-poly(n-propyl methacrylate) as a model system, molecular parameters characterizing the copolymer were shown to yield the growing tendency of surface segregated or phase coherent profiles upon heating. Neutron reflectivity measurements were performed for the copolymer films to be compared with theory. Phase coherence prior to bulk ordering temperatures and subsequent decay lengths observed for the given system were shown to be in agreement with theory.