The reactive crystallization was carried out by adding Na2CO3 solutions to Ca(OH)2 suspended solutions. This system is included in the causticizing reaction in the kraft pulping method. If high quality of calcium carbonate is obtained and used directly as fillers in papers, it is very meaningful for paper industry. It is known that aragonite is especially useful as fillers. However, only few works were performed for the crystallization behaviors in this system previously. It was cleared in this study that aragonite and calcite precipitate and vaterite do not appear at 323K. At the beginning of the crystallization only calcite precipitated, however, aragonite crystals started to precipitate when the molar ratio of sodium carbonate to calcium hydroxide got to be larger than about 0.3. After then, the composition of aragonite increased and attained to the constant value. This indicates that the highly alkaline solutions is advantageous for the nucleation of aragonite. The crystallization of aragonite crystals was accelerated by decreasing the addition rate of sodium carbonate solution. Furthermore, with the increase of the solution volume and the stirring rate the crystallization of the amount of aragonite was increased. These results indicate that low supersaturation is advantageous for the selective crystallization of aragonite. After the addition of sodium carbonate solutions the aragonite transformed to calcite crystals . The mechanism of the transformation was cleared to be “solution-mediated”. Further more, it cleared that the yield of aragonite crystals depends on temperature and the optimum temperature was found in this work.