The shock compression response of nano-Fe powder(∼25nm) pressed to ∼35% theoretical maximum density was determined based on shock stress and wavevelocity measurements using piezoelectric stress gauges. The obtained data show a discontinuity in shock wavevelocity plotted against particle velocity and an inflexion in specific volume from compression to expansion with increasing shock stress. It is found that the Hugoniot of 25nm Fe powder cannot be fully described using analytical models that are otherwise capable of predicting the Hugoniot of micron-sized powder or highly porous materials.