The effect of autoclaving hydrothermal treatment on the characteristics of plasma-sprayed hydroxyapatite (HA) coatings on the Ti-6Al-4V substrate was investigated. The heating temperatures were 100°C, 150°C and 200°C with ambient saturated steam pressure in an autoclave. On the basis of quantitative analysis of crystallinity using x-ray diffraction (XRD), hydrothermal treatment was found to be effective for increasing the crystallinity and phase purity of the HA coatings. The prominent and sharp OH− and PO4 3− peaks detected from x-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra demonstrate a superior crystallized integrity of hydrothermal-treated HA coatings through the incorporation of water vapor. Moreover, the significant presence of OH− peak in XPS spectra represents a replenishment of water molecules which tends to reduce the dehydroxylation state of as-sprayed HA coatings. From the observation of microstructures, crystallized HA was found to diminish the spraying defects of hydrothermal HA coating layers, and finely-crystallized HA crystals, with a Ca/P atomic ratio of 1.67, were observed through transmission electron microscopy (TEM). Hydrothermal treatment could induce a low-temperature crystallization process, and the saturated steam pressure is thought to be a factor which reduces the activation energy and accelerates the HA crystallization. Experimental evidence confirmed that the ambient saturated steam pressure plays an important role in lowering heating temperatures and promoting HA crystallization.