The refinement of alpha- and gamma -hematite powders was examined using a nozzle cavitation in a mixture of their starting powders and a water. Their mean diameters were reduced from 1.01 mun to 0.84 mum for alpha -Fe2O3 and from 1.48 mum to 0.41 mum for gamma -Fe2O3, which was not remarkable compared to TiO2 and ZrO2 powders previously reported. It was suggested that these low size reductions were caused by the gentle necks between the primary particles in these powders.