The present work was focused on the dynamic characterization of nanoparticles production under continuous flow in a droplet-based spiral-shaped microreactor. A novel drainage design was incorporated into the microreactor channel network for eliminating the commonly encountered problems of clogging and coalescence of microdroplets containing nanoparticles in the microchannel. Through precise manipulation of droplets formation for investigating the precipitation process of iron oxide, one-to-one pairing and fusion of reactant droplets were obtained and monitored in a real-time manner through digital imaging, where optimised flow rate ratios (in a range of 0.3-0.5) of aqueous phase to oil phase were applied. The nanoprecipitation process was observed and further analysed, indicating that the formation of precipitates was determined by the diffusion of OH- after coalescence of reactants droplets. By characterising the Fe3O4 nanoparticles produced, it demonstrated that smaller droplets were beneficial to obtaining high quality nanoparticles in terms of size control and size distribution. Moreover, the kinetics of the nucleation process was investigated through statistic calculation. Initial results suggested that heterogeneous nucleation occurring inside droplets was likely due to the presence of impurities-mediated (e.g. surfactants) or collision-induced nucleation events. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.