CaHPO4 particles were synthesized in oil-in-water (o/w) system and water-in-oil (w/o) system with membrane microdispersion mixing technique. The particles of CaHPO4 were prepared using two kinds of micro-filtration plate membrane modules with different active areas as a dispersion medium. The working system of calcium acetate aqueous solution/n-butanol with phosphoric acid as the transferred solute was applied. The influence of the reactant concentration and the operation conditions on the performance of this process was investigated. Particle morphology was observed by scanning electron microscope, and the particle-size distributions were measured by the photo-imaging method. The results showed that the CaHPO4 particles with narrow size distribution could be prepared using membrane dispersion method in the o/w or w/o system. The average sizes were affected by the module structure, residence time, reactant concentration and phase ratio of the aqueous phase to the oil phase. The particles with the size ranging from 0.2 to 0.6 mu m were prepared in the module with 12.5 mm(2) of active membrane area. When the active area was 400 mm(2), the particles were in range of 10-20 mu m. (c) 2005 Elsevier B.V. All rights reserved.