Very thin (less than 100 μm) grain-oriented silicon steels are known to have lower iron loss than iron-based amorphous materials. It is possible to reduce the iron loss of very thin grain-oriented silicon steels further by applying new magnetic domain refining techniques. One method for magnetic domain refining is to control the grain size. We observed the magnetic domains and measured the iron loss of very thin grain-oriented silicon steels that had various grain sizes. We controlled grain size in samples to be from 0.25 mm to 2.3 mm without changing Bg. This experiment shows that the magnetic domain width can be made narrower by decreasing the grain size, which reduces the eddy current losses. On the other hand, samples with larger grain size have lower Hc and lower hysteresis losses. Therefore there is an optimum grain size for the lowest iron loss of very thin grain-oriented silicon steels.