An investigation of the effect of moisture content on the bulk density of iron ore fines has been carried out. Particle size distribution, density of agglomerates and angle of repose in both wet and frozen states were measured as a function of moisture content. Based on the experimental results, the mechanisms governing the relationship between bulk density and moisture content, i.e. the density of agglomerates, the inter-particle friction, tightened size distribution and agglomerate deformation, were quantitatively analysed. The results suggest that when moisture content is small, less than 6.5% for the ore fines considered the mechanism of agglomerate deformation can be ignored, and the mechanisms of inter-particle friction and tightened size distribution result in a decrease in bulk density. When moisture content is large, the mechanism of agglomerate deformation becomes effective too, which, together with the mechanism of inter-particle friction, results in an increase in bulk density after it reaches a minimum value. The density of agglomerates also affects the relationship between bulk density and moisture content, resulting in a decrease in bulk density as moisture contents increases, like the mechanism of tightened size distribution but with a smaller effect. The findings should be useful to the development of strategies for controlling the bulk density of ore fines in practice. (c) 2006 Elsevier B.V. All rights reserved.