Microstructures of the interface between plasma-spray coated hydroxyapatite (HAp) and Ti-alloy substrate have been investigated mainly by means of transmission electron microscopy. The results obtained are as follows; HAp coating exhibits a high crystallinity. The interface of HAp/Ti-alloy is winding at an interval of about 100 mum and thickness of HAp coating is about 50 mum. Al2O3 abrasives remained in both HAp coating and Ti-alloy substrate around the interface. Amorphous phase was not observed at the interface. The grain size of Ti-alloy in the vicinity of the interface is about 10 to 100nm in diameter, while that far from the interface is about 3 mum in diameter. Miniaturization of Ti-alloy grain around the interface was occurred by plasma spraying. There is no specific crystallographical orientation relationship between HAp and Ti-alloy at the interface, indicating bonding of the interface is essentially mechanical one such as an anchor effect.