Mg²⁺ and bovine serum albumin (BSA) Langmuir monolayers were used as effective crystal nucleation and growth modifiers and template to control the crystallization of CaCO₃. The polymorph and morphology of crystals were characterized using scanning electron microscopy (SEM), and X-ray diffraction (XRD). The results demonstrated 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 produced at the interface of film/solution. With the increasing of the molar ratio of Mg²⁺ to Ca²⁺, the polymorph of calcium carbonate obtained undergoes an evolvement from calcite to vaterite and aragonite. 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 crystals obtained were all aragonite. In the systems of Mg²⁺-BSA Langmuir monolayers, the presence of Mg²⁺ was helpful for the formation of aragonite. The possible formation mechanisms of CaCO₃ in different systems were discussed in the paper.