Needlelike CaCO3 particles consisting mainly of aragonite phase with a little calcite content were synthesized by carbonation in high-gravity environment generated by a rotating packed bed (RPB) reactor with H3PO4 as additive. The influence of the concentration of additive, CO2 flow rate, reaction temperature and rotating speed of RPB on the formation of needlelike CaCO3 particles was discussed. The results showed that the addition of H3PO4, high reaction temperature, low CO2 flow rate and appropriate rotating speed of RPB favored the formation of needlelike CaCO3 particles. It was concluded that H3PO4 reacted with Ca(OH)(2) to form hydroxyapatite (HAP), which served as crystal nucleus leading to the formation of aragonite-type CaCO3. The total carbonation time in RPB was about 1/3 of that reported by other methods. (C) 2004 Elsevier B.V. All rights reserved.