A microporous tube-in-tube microchannel reactor (MTMCR) was Successfully adopted to prepare hydroxyapatite (HAP) nanoparticles. The rodlike HAP nanoparticles with a mean size of 58 nm, a specific Surface area of 49.32 m(2)/g, and a narrow size distribution were obtained in ail MTMCR under a high throughput of 3 L/min. The mean particle size sharply decreased with increasing the continuous phase flow rate, while first decreased and subsequently increased with increasing the dispersed phase flow rate and the reactant concentration. The extension of the mixing distance led to the initial rapid and following slight decrease of the mean particle size. The size of HAP nanoparticles was also strongly dependent oil the micropore size on the surface of inner tube. Small micropore size was beneficial for producing small particles. For comparison, HAP nanoparticles were also prepared in a stirred tank reactor (STR) and T-junction microchannel reactor (TMCR), clearly exhibiting the advantages of the MTMCR over the STR and TMCR due to the achievement of uniformly smaller HAP nanoparticles and a high throughput for industrial production.