In our previous work, a membrane dispersion micro-structured reactor has been successfully applied to the preparation of nano-particles. It is a general requirement to model the process for large scale production. The classic precipitation theory, population balance model (PBM), as the main model has been tested in this work, and it has been found that the classic model by introducing a parameter of mixing scale can reliably simulate the process. The model is used to calculate the precipitation of BaSO4 nano-particles. The diffusion of reactants. nucleation, growth and particle agglomeration have been considered in the model. The evolutions of supersaturation ratio and particle number density distribution with the model can be predicted which help us to understand the effect of mixing scale on the three stages of pre-nucleation, clearly, nucleation and growth. The particle size and its distribution are also calculated and the results are in good agreement with the experimental data. (c) 2009 Elsevier B.V. All rights reserved.