Crystal growth mechanism for hydroxyapatite under pseudophysiological conditions was investigated using hydrothermally synthesized single crystal as a seed. In situ surface observation and measurements of step velocity indicated that the rate determining process of the growth was incorporation of the growth unit into the crystal lattice. Step kinetic coefficient on (1010) face was the order of 10-4 cm/s, which was 2-3 orders smaller than that for soluble inorganic crystals and the same order as that for macromoiecular protein and virus crystals. The edge free energy of nucleated island on (0001) face, which was estimated using phase shift interferometry, also showed a same order as that for protein crystal. These results strongly suggest that a growth unit of hydroxyapatite was a form of cluster and not ionic species. This hypothesis was confirmed by dynamic light scattering measurement of the particle size in the solution in which hydroxyapatite grew. Around 0.8 nm in diameter of calcium phosphate clusters were detected by light scattering measurement. In the crystal structure of hydroxyapatite, two kinds of Ca9(PO4)6 chiral clusters called C0 and C50, both of which are about O.8 nm in diameter, can be defined. A growth model based on the stacking of these chiral clusters was presented. Hydroxyapatite can grow only by two kinds of stacking. One is stacking of either C0 or C50 clusters forming a primitive lattice with two-dimensional hexagonal packing. The other is stacking of C0 and C50 alternatively with hexagonal closed packing.