In the hydrothermal synthesis process of layered octosilicate ilerite, seed crystals were added to the starting solution (water glass) for control of the crystallization rate. Octosilicate platelets of 4.4 mu m and their ground products with a size less than 1 mu m were used as the seed. The crystallization rate increased with an increase in the additional amount of platelet seed. The crystalline products were obtained even if the seed had a relatively low crystallinity. The size of products depended strongly on both the additional amount and size of seed. When the seed-loading mass ratio of seed to the starting solution was 0.01, the crystal growth was a maximum and the size of products was about twice of that of seed. However, in the use of the ground seed, a finer product was obtained at the optimum seed-loading mass ratio due to increase in the number of seed crystals. To control crystal growth for reducing synthesis time and for obtaining crystals of a specific size, both number and size of seed crystals must be optimized because the silanol groups on the edge of seed crystals added become new reaction sites of crystallization.