One method for increasing productivity of a microreactor is presented without demanding numbering-up processes by using a newly proposed microporous tube-in-tube microchannel reactor (MTMCR) with a high throughput of 9 L/min. Barium sulphate nanoparticles with an average size of 37 nm and a narrow size distribution were successfully produced in MTMCR. The size of barium sulphate particles was strongly dependent on the flow rate of the reactants in the microchannel and high flow rate was beneficial for producing small particles. Furthermore, it was possible to control the average particle size in the range from 30 to 150 nm by changing the reactant concentrations and the micropore size on the surface of the inner tube. The average particle size decreased by increasing the total flow rate and reactant concentrations or decreasing the micropore size. The dispersed-phase flow rate and mixing distance exhibited little effects on the particle size. The proposed microreactor concept was confirmed to be valid and promising for the industrial production of nanoparticles. (C) 2009 Elsevier B.V. All rights reserved.