The manufacturing process of precipitated calcium carbonate (PCC) is comprised of three steps. First, calcium oxide is obtained by calcination of limestone. The calcium oxide then turns into calcium hydroxide through a reaction with water (hydration process). Finally, PCC is manufactured by a reaction between the calcium hydroxide and carbonate ions. The hydration process is an exothermic reaction and the hydration activity of calcium oxide is expressed as the heat generated during the hydration process. However, at present there is no standard for the measurement method of the hydration activity of calcium oxide. Therefore, in the present study, we investigated the effect of hydration activity of calcium oxide on the manufacturing process of PCC in order to propose a Korean standard for the measurement of hydration activity of calcium oxide. Based on a comparison of the formation yield of PCC with the hydration activity of calcium oxide measured by various methods, it was verified that the characteristics of PCC are strongly affected by the amount of heat generated for a period of 30 seconds after hydration commences. Therefore, the measurement of hydration activity of calcium oxide for manufacturing PCC should be performed during the initial 30 seconds of hydration. We also confirmed that the formation yield of aragonite PCC increases with increasing hydration activity of calcium oxide. In contrast, calcite PCC decreases.