This paper reports on the effect of pH on polymorphism of calcium carbonate (vaterite) formed in the CaCl2-Na2CO3-H2O reaction system. The initial pH of CaCl2 aqueous solution was adjusted to the initial pH 2.0-11.3 using HCl and NH3 aqueous solution. The initial pH was 6.5 without pH adjustment. When the reaction was carried out, amorphous calcium carbonate (ACC) was formed at the early stage of the reaction and crystallized to calcite and vatrite after 3 min of stirring. Formation ratio of vaterite at this time was 60%. Single-phase vaterite was formed at initial pH 2.0-2.5. Further, focusing on the pH during ACC crystallization. When HCO3- ions were 80% or more during ACC crystallization, single-phase vaterite was formed.