Researches on precipitation of calcium from seawater have been carried out using CO2. The effect of pH value on calcium removals from seawater was investigated. It was found that it is easier to remove calcium from seawater at a higher pH value, and the removal rate of calcium can reach 60% to 90% when the pH value is from pH 8.0 to pH 9.0. The precipitated solids were characterized using X-ray diffraction (XRD) analysis. The results show that they consist of CaCO3·H2O at pH 8.0 to pH 8.7, of CaCO3·H2O and MgCO3·3H2O at more than pH 9.0.