Aragonite CaCO3/polymer thin film composites were successfully obtained by using cooperative effects of chitosan, poly(aspartate), and MgCl2. The structures of these composites were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The combination of poly(aspartate) and the magnesium ion induced higher aragonite fractions in the thin films on the chitosan matrix, while the use of poly(acrylic acid) with Mg2+ gave lower aragonite fractions.