A series of the Mg-substituted nano-hydroxyapatite (Mg-n-HA) nanopowders were synthesized by chemical co-precipitation method under different conditions, including two reaction temperature of 70 degrees C and 40 degrees C, and two water-soluble macromolecules of carboxyl methyl cellulose and poly(acrylic acid). The influence of magnesium content, reaction temperature and macromolecules on the microstructure, crystallinity and solubility of Mg-n-HA were characterized using SEM, XRD, FTIR techniques and soaking in simulated body fluid (SBF) solution. The results showed that needle-like Mg-n-HA nanoparticle with Mg being successfully incorporated in HA lattice could be synthesized, and there was no obvious morphology difference among the Mg-n-HA nanoparticles synthesized under different conditions. However, the crystallite sizes, cell parameters and crystallinity evidently decreased with the increasing magnesium content, the lowering reaction temperature and the introducing of macromolecules. Moreover, the solubility in SBF soaking was greatly improved in comparison with n-HA due to their lower crystallinity. (C) 2014 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.