High resolution transmission electron microscopy (HRTEM) and X-ray energy dispersive spectroscopy (EDS) were applied to study the structure of hydroxyapatite (HAP) coatings deposited by plasma spraying on different substrates (Cu, Cr, Ni, NaCl, BaF2). Coatings were obtained with a 13.56 MHz RF argon plasma at 1 Pa by placing a HAP target as the RF electrode. The typical rate of coating formation was about 1.0 - 1.5 nm/s and the total thickness was 350 - 400 nm. For all coating conditions and substrates investigated here, coatings are constituted of single-phase HAP polyhedral nanocrystals of 3-20 nm size randomly oriented to each other and without texture relatively to the substrate.