This paper deals with the mechanism of crystal growth of layered octosilicate, called ilerite, in the hydrothermal synthesis in the presence of seed crystals. The seed crystals grew efficiently at the initial period of the crystallization process; however, the crystal growth rate decreased with an increase in their size. In addition, the seeding promoted the production of crystal nuclei and their growth, resulting in an increase in the formation of finer particles. The pH value of the solution increased as the progress of the synthesis reaction, resulting in inhibition of the crystal growth. In order to enhance the crystal growth, the hydrothermal synthesis with controlling the pH value was carried out. By decreasing the pH value to the initial one, the yield of ilerite reached over 80%, however, the particle size of products increased slightly because of the production of crystal nuclei and their growth.