A micro-chemical process was applied to prepare nanometer-size hydroxy apatite (HA) particles with various shapes and sizes so as to provide high surface activities, where solutions of reagent-grade calcium nitrate tetrahydrate and diammonium hydrogen phosphate were introduced in a micro reactor. The particles were characterized by X-ray diffractometry, transmission electron microscopy, or inductively coupled plasma photometry. Particle size and shape were dependent on not only pH and concentration of the mixing solutions but also the flow rate. Dilute solutions containing 3 mM Ca(II) and 5 mM P(V) yielded spherical HA particles, whose average particle size increased from about 3 nm to 40 nm as the flow rate increased from 0.3 to 3 ml·min -1, where pH and the temperature of the reaction system were respectively maintained at 10 and at 40°C. Concentrated solutions with 50 mM Ca(II) and 30 mM P(V) yielded rectangular plate-like HA particles, 300 nm long and 50 nm wide, when pH was kept at 8, while agglomerations of smaller HA particles of about 20-50 nm were obtained when pH was kept at 10.