Nanoparticles have been produced by a T-junction microchannel device in our previous work (Li et al., Langmuir. 2008;24:4194-4199). As a scaling-up strategy, pore array microstructured devices were designed to prepare nanoparticles in this article. H2SO4 and BaCl2, respectively, in two phases to form BaSO4 nanoparticles was used as a test system. The characteristics of a well controlled liquid-liquid two-phase flow in the pore array, microstructured devices were presented. Nanoparticles with small size and good dispersibility were produced through drop or disk flows in the microstructured devices. The influence of mass transfer and chemical reaction on interfacial tension and flow patterns was discussed based on the experiments. Meanwhile, the effect of the two phase flow patterns on the nanoparticle,size was discussed. It was found that the increase of the amount of mass transfer and chemical reaction could change the flow patterns from disk flow to drop flow. The droplet diameter could be changed in a wide range. Flow patterns could be distinguished based on the measured interfacial tension it? different concentrations. The prepared nanoparticles were ranged from 10 nm to 30 nm. Apparently the particle size was decreased with the increase of the droplet size in both the drop flow region and the disk flow region whereas it had a reverse trend in the transition region. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 3041-3051, 2009