Nanosized hydroxyapatite (HA) powders were synthesized via a unique oil-in-water emulsion processing route using petroleum ether as the oil phase, KB6ZA as the nonionic surfactant phase, and aqueous CaCl2 solution as the water phase. An (NH4)(2)HPO4 solution was added to the oil-in-water emulsions to form hydroxyapatite. The resulting HA powders as examined by transmission electron microscopy are nanosized and crystalline. Very little growth in particle size of RA is observed upon calcination at 650 degrees C for 6 h. In contrast, the HA powders prepared by directly reacting (NH4)(2)HPO4 With CaCl2 in an aqueous solution and in a micellar system containing 3.2 wt % KB6ZA nonionic surfactant are much less crystalline than those formed in the oil-in-water emulsions. Moreover, the KA powders underwent extensive growth in crystallite and particle sizes when calcined at 650 degrees C for 6 h. A mechanism for the formation of nanocrystalline HA is proposed for the oil-in-water emulsions containing 2-11 wt % petroleum ether. The complexation of Ca2+ ions by oxyethylene groups of the nonionic KB6ZA surfactant on the surface of emulsion droplets constitutes numerous sites for forming nanosize HA crystallites. This unique oil-in-water emulsion process can produce not only nanocrystalline HA powders with high yield, but also uses a much smaller amount of oil and surfactant phases than that required by the water-in-oil microemulsion process.