Hydroxyapatite (HAp) particles were synthesized by solid-state reaction and wet chemical reaction, and were characterized in terms of their chemical composition, disordered structure and in vitro biodegradability. An X-ray diffraction study revealed that the prepared HAp particles were composed of single phase HAp, while 1D and 2D solid-state NMR analysis showed that they consisted of not only crystalline HAp but also a disordered phase. An in vitro biodegradability test showed that wet chemically derived HAp particles were degraded quicker than commercially available HAP-100. The in vitro biodegradability was discussed by using a structure model for nano-crystalline HAp, in which the nano-crystals consist of a crystalline HAp core covered with a disordered surface layer (core-shell model). Although the specific surface area was the predominant factor on the rate of Ca ion dissolution, the disordered surface layer enhanced the release of Ca ions in the initial stage within 1 min, while the crystalline core of HAp also gave different release rate of Ca ions, depending on the chemical distribution in the P (V) environment.