Decreasing the size of Li2CO3 powder has benefits in its applications for drug delivery and materials manufacture. To this end, selecting a water-ethanol mixture as solvent, we investigate the precipitation reaction between LiOH and CO2 in a microfiltration membrane dispersion microreactor. The results show that both interphase mass transfer rate of CO2 and the solubility of Li2CO3 play important roles in a successful Li2CO3 nanoparticles preparation. The ratio of the solubilities of LiOH and Li2CO3 reaches nearly two orders higher than that in water at the mass fraction of ethanol (phi) being around 0.9, and 0.870 or higher of phi can guarantee obtaining nanoparticles with narrow size distribution (CV < 0.3). The particle size is sensitive to phi and the flux of liquid/slurry feed, and the increases of which can lead to the decreasing of particle size in the range from 150 to 30 nm. The usage of LiOH slurry with content up to 10 wt % can increase the particle size a little but does not affect the uniformity of nanoparticles in morphology. Such a Li2CO3 nanoparticles preparation process could be green with water as the only byproduct and discharge.