In the reaction crystallization of calcium carbonate using a batch reactor system with aqueous CaCl2 and Na2CO3 at room temperature, the selective polymorphic control of crystalline calcium carbonate into calcite was achieved by adjusting the operating conditions. The polymorphic ratio of calcite and vaterite nucleated from amorphous calcium carbonate (ACC) was significantly influenced by the initial supersaturation and solution pH, thereby resulting in a different phase transformation rate for the product powder from unstable vaterite to stable calcite. With a high supersaturation and low solution pH, the phase fraction of calcite decreased owing to the favorable transformation of ACC into vaterite during the early stage of the crystallization. Thereafter, the phase fraction of calcite increased as a result of the transformation of vaterite into calcite as the aging progressed. However, with a low supersaturation and high solution pH, calcite was exclusively observed throughout the reaction time without any recrystallization of vaterite. The decrease in the polymorphic ratio of vaterite in the product powders with the reaction time indicated the disappearance of vaterite due to its dissolution at a low initial supersaturation, which matched well with the morphological change in the polymorphs obtained by a scanning electron microscopy analysis. Meanwhile, at a high initial supersaturation, the polymorphic changes from vaterite to calcite did not produce any morphological transformation.