Simulations were performed for seeded batch cooling crystallization. In industry, partial seeding is often utilized, where a small amount of seed crystal is added to trigger secondary nucleation and grown secondary nuclei are obtained as the product. Partial seeding was investigated by computer simulation in this study. First, the coefficient of variation (CV) of the product crystal size distribution (CSD) was proven to take two local minima, one in the partial seeding range and the other in the range in which the product was mainly composed of seed-grown crystals. The local minimum point in the partial seeding range was considered as an optimum condition for partial seeding. Then, CSDs of grown seed crystals and nuclei were simulated at the optimum seed-loading ratio. As a result, the product was found to be composed mainly of grown secondary nuclei induced by grown seed crystals and grown secondary nuclei themselves. Partial seeding performed at the optimum seed-loading ratio yielded a reasonably good product of unimodal size distribution. Finally, the optimum seed-loading ratio was correlated with the cooling rate and seed crystal size and was found to depend on the cooling rate and more strongly on the seed crystal size. As the cooling rate was decreased, the minimum CV was shown to decrease and the mean size at the optimum seed-loading ratio was shown to increase.