Highly-crystallized hydroxyapatite (HA) can be precipitated during heat treatment in high-pressure steam at 300 degrees C on an anodic titanium oxide film containing Ca and P (AOFCP), which has been electrochemically formed on a titanium substrate prior to the hydrothermal treatment. Factors affecting the precipitation, such as a percentage of distilled water in the autoclave and additives in the AOFCP, were evaluated by scanning electron microscopy. Ca2+ and PO43- ions were leached from the AOFCP into a water layer covering the film surface, and nucleate HA heterogeneously on the porous TiO2 matrix of the AOFCP which was made by the ion leaching. The morphology of the precipitated crystals was significantly affected by the water volume ratio because the concentrations of the Ca2+ and PO43- ions varied depending on the thickness of the water layer. The amount of the precipitation decreased on the AOFCP which was formed in the solution containing a small amount of Mg2+ ions or formed on Ti-6Al-4V alloy instead of titanium.