Mg² and bovine serum albumin (BSA) Langmuir monolayers were used as effective crystal nucleation, growth modifiers and template to control the crystallization of CaCO₃. Scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to characterize the polymorph and morphology of crystals obtained at different experimental conditions, respectively. The results indicated that various morphologies such as abacus-bead-like particles, spherical-shaped particles, wood block-like particles, pignut-shell-like particles and the rolling pole shaped particles have been formed at the interface of air-solution. The polymorph of calcium carbonate obtained undergo an evolvement from calcite to vaterite and aragonite with increasing of the molar ratio of Mg² to Ca², which indicated that the ability of Mg² to induce the formation of aragonite was enhanced as the molar ratio of Mg² to Ca² increased. When the molar ratio reached 3, the samples obtained were all aragonite phase of calcium carbonate, which suggests that the presence of Mg² of subphase solutions was helpful for the formation of aragonite phase in the systems of Mg²-BSA Langmuir monolayers. The possible formation mechanisms of CaCO₃ in different systems were discussed in the paper.