In order to improve biocompatibility of Ti metal substrates, 1-mu m-thick nanostructured hydroxyapatite (HAp) coatings were deposited on the substrates through aerosol deposition, which sprays HAp powder with an average particle size of 3.2 mu m at room temperature in vacuum. The original HAp particles were fractured into nanoscale fragments to form highly dense coating during the deposition process. Density of the HAp coating was 98.5% theoretical density (TD). Transmission electron microscopy observation revealed that the as-deposited coating consisted of HAp crystallites with average grain size of 16.2 nm and amorphous phase. Tensile adhesion strength between the coating and the substrate was 30.5 +/- 1.2 MPa. Annealing up to 500 degrees C in air increased crystallinity and grain size in the coating without any delamination or crack according to X-ray diffraction analysis and electron microscopy. MTS assay and alkaline phosphatase activity measurements with MC3T3-E1 preosteoblast cell revealed that the biocompatibility was greatly improved by postdeposition heat treatment at 400 degrees C in air due to well-crystallized HAp with average grain size of 29.3 nm. However, further heat treatment at 500 degrees C deteriorated biocompatibility due to rapid growth of HAp grains to average size of 99 nm. Cross section of the coating on a commercially available Ti dental implant revealed full coverage of the surface with HAp.