In this work, nano-CaCO3 particles with tunable size have been synthesized via CO2/Ca(OH)(2) precipitation reaction in a microporous tube-in-tube microchannel reactor (MTMCR) with a throughput capacity up to 400 L/h for CO2 and 76.14 L/h for liquid. The overall volumetric mass-transfer coefficient (K(L)a) of CO2 absorption into Ca(OH)(2) slurry in the MTMCR has been deduced and analyzed. To control the particle size, the effect of operating conditions including initial Ca(OH)(2) content, gas volumetric flow rate, liquid volumetric flow rate, micropore size, and annular channel width was investigated. The results indicated that the mass transfer in the MTMCR can be greatly enhanced in contrast with a stirred tank reactor, and the particle size can be well controlled by tuning the operating parameters. The nano-CaCO3 particles with an average size of 28 nm and a calcite crystal structure were synthesized, indicating that this process is promising for mass production of nanoparticles. (C) 2014 Elsevier B.V. All rights reserved.