The mechanism of biomineralization of bone-like apatite on synthetic hydroxyapatite (HA) has been investigatedin vitro, in which the HA surface was surveyed as a function of soaking time in simulated body fluid (SBF). In terms of surface structure by transmission electron microscopy with energy-dispersive X-ray spectrometry, the HA whose Ca/P atomic ratio was 1.67 revealed three different characteristic soaking periods in SBF, i.e. the first soaking period, in which the HA surface increased the Ca/P ratio up to 1.83 to form an amorphous phase of Ca-rich calcium phosphate; the second soaking period, in which the HA surface decreased the Ca/P ratio up to 1.47 to form an amorphous phase of Ca-poor calcium phosphate; and the third soaking period, in which the HA surface gradually increased the Ca/P ratio up to 1.65 to eventually produce the bone-like nano-cerystallites of apatite, which grew forming complex crystal assemblies with a further increase in immersion time. Analysis using electrophoresis spectroscopy indicated that, immediately after immersion in SBF, the HA revealed a highly negative surface potential, which increased to reach a maximum positive value in the first soaking period. The surface potential then decreased to again reach a negative value in the second soaking period and thereafter converge to a constant negative value in the third soaking period. This implies that the HA induces biomineralization of apatite by smartly varying its surface potential to trigger an electrostatic interaction, first with positive calcium ions and second with negative phosphate ions in the SBF.