Free-standing films of calcium phosphates exhibit many favorable properties for tissue engineering. In this work, a thin film of calcium phosphate is prepared in a liposome suspension using the method of ammonia gas diffusion. The thickness of the film is about 10 μm, and the lateral dimensions are on the length scale of millimeter. The results of powder X-ray diffraction and transmission electron microscopy show that the thin films contain the mineral phases of hydroxyapatite and octacalcium phosphate (OCP). Using solid-state NMR spectroscopy, in particular the technique of heteronuclear correlation spectroscopy with variable contact time, the major crystalline phase of the thin film has been confirmed to be OCP.