Very thin (less than 0.1 mm thick) grain-oriented silicon sheets are known to have löwer iron losses than iron-based amorphous materials. It is possible to reduce the iron loss of very thin grain-oriented silicon steel further by applying new magnetic domain refining techniques. One method of domain refinement is through control of the grain size. We observed magnetic domains and measured the iron loss of very thin grain-oriented silicon steel sheets with small (less than 1 mm) grains. These experiments showed that the domains can be made narrower by decreasing the grain size, which reduces eddy current losses. On the other hand, materials with larger grain sizes have lower Hc values and lower hysteresis losses.