CaCO₃ was synthesized under various basic conditions through carbonation reaction. CO₂ gas was blown into calcium hydroxide solution and the reaction pH was controlled constantly with NH₄OH and KOH aqueous solution at specific levels which are 12.5, 11.5, 10.5, 9.5 and 8.5. Each sample prepared was analyzed with X-ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and BET surface area. And concentration of Ca²⁺ ion of filtered solution was measured with inductively coupled plasma-atomic emission spectrometer (ICP-AES). It was found that the pH affect on the crystallization when the NH₄OH was used as pH control additives. In the case of the NH₄OH addition, the particle size grew larger with decreasing of pH and increasing of reaction time. Ammonia induced the calcium carbonate dissolution, so that the small nuclei were dissolved and re-precipitated on the surface of larger nuclei. Because of the effect of ammonia, calcite particle prepared grew gradually as the reduction of pH. On the contrary, there are not influences of pH for precipitation of calcium carbonate when the KOH solution was added. It was suggested that the additives to control pH affect on the precipitation of calcium carbonate rather than reaction pH.