Ferroelectric (FE) HfO 2 has a higher coercive field (Ec) than that of perovskite. High Ec guarantees a robust FE state, but it also implies a relatively slow switching speed under the same external field. Here, we propose a control of grain size in Si:HfO 2 which would lower the Ec, thereby, increase the switching speed. In order to achieve the goal, we studied and optimized Si-contents and post-anneal condition for nano-grain sized Si:HfO 2 . We successfully demonstrated that Si:HfO 2 consists of controlled nano-grains with a FE property of Ec ~0.5MV/cm, which is a half of the ordinary Si:HfO 2 , and the domain switching speed reaches ~3 times faster than that of ordinary grain sized Si:HfO 2 . The domain dynamics is explained by the intrinsic domain-wall growth mechanism, which means no significant defect generations during the nano-grain formation.