The effect of power input, solid content and ionic strength of liquid on the kinetics of de-agglomeration of acicular goethite nano-particles in ultrasonic comminution device has been investigated. It has been found that the pattern of de-agglomeration is independent of power input. Initially large aggregates are broken by fragmentation and as the process progresses the primary particles are gradually eroded from the surface of those large aggregates. The breakage of large aggregates was described by size-energy model and the model describing the generation of primary particles was developed. The increase of solid concentrations in the suspension (up to 20 wt.%) leads to an increase of the efficiency of both breakage of large aggregates and formation of fine particles. The ionic strength and solid concentration have practically no effect on mechanism of de-agglomeration but they affect the morphology and rheology of the suspensions of goethite nano-powder. (C) 2008 Elsevier B.V. All rights reserved.