The microstructure of nano-sized hydroxyapatite (HAp) powders coating layer on ZrO$_2$ substrate was investigated, which was formed by plasma spray process. The nano-sized HAp powders were successfully synthesized by precipitation of Ca(NO$_3$)$_2$$.$4H$_2$O and (NH$_4$)$_2$HPO$_4$ solution. The HAp coating layer with thickness of 150∼250 $mu extrm{m}$ was free from the cracks at interfaces between the coating and ZrO$_2$ substrate. In the plasma sprayed HAp coating layer, the undesirable phases were not found, while in the HAp coating layer heat-treated at 800$^{circ}C$, TTCP, and ${eta}$-TCP phase were detected as well as HAp phase. However, at 900$^{circ}C$, they were completely disappeared. At 1100$^{circ}C$, XRD analysis revealed that the coating layer was composed of the highly crystallized HAp.