In this paper, a new method to synthesize BaSO4 nano-particles in O/W system with filtration dispersion technology was developed. The nano-particles of barium sulfate were prepared using a type of filtration dispersion device with a plate porous disc as a dispersion medium. Working systems of barium chloride aqueous solution/30% TBP (in kerosene) with sulfuric acid were applied. The influence of the reactant concentration and the operation conditions were investigated. The morphology of the particles was observed by SEM, and the particle-size distributions were measured. The nano-particles prepared in O/W system were compared with those prepared by dispersing aqueous barium chloride solution into aqueous sodium sulfate solution with the same filtration dispersion device. The results showed that mono-dispersed BaSO4 particles can be prepared using the filtration dispersion method in O/W systems. The surfaces of all the particles were smooth. The mean diameter of the particles was in the 100-400 mn range and decreased with the increase of flux of the two phases. The ratio of the two phases greatly affects the morphology of particles because of the excess concentration of [SO42-]. The particle size was also much smaller when the mixed two feeds were aqueous solutions. (c) 2005 Elsevier B.V. All rights reserved.