Hydroxyapatite (HAP) nanopowders were prepared by the combination of chemical reaction in a tube-in-tube microchannel reactor and hydrothermal crystallization process.The influences of reactant flow rate ratio，microchannel height，hydrothermal temperature and time on the preparation of HAP nanopowders were systematically investigated.The results showed that increasing reactant flow rate ratio or decreasing microchannel height led to the decrease of the particle size of HAP nanopowders.The crystallinity and thermal stability of HAP were enhanced with the increase of hydrothermal temperature and time.The reactant flow rate ratio and hydrothermal temperature were the main factors that influenced the preparation of HAP nanopowders.The uniform，well-crystallized and rod-like HAP nanopowders with a mean particle size of about 80 nm could be prepared under the experimental conditions of reactant flow rate ratio of 5∶1，microchannel height of 250 μm，hydrothermal temperature and time of 220℃ and 4 h，respectively.