This study aims to explore phase transformation in plasma-sprayed hydroxyapatite (HA) + 10 wt% ZrO2-8-mol%-Y2O3 composite coating, using separately prepared HA and ZrO2-8-mol%-Y2O3 coatings as a control. Changes in the phase and chemistry of the coatings are characterized by X-ray diffractometry, with lattice-constant measurement (Cohen's method), and by transmission electron microscopy. Experimental results show evidence of diffusion, in the liquid state, of calcium ions from the HA matrix into the ZrO2. This behavior causes the formation of the following structural features in the composite coating: (I) a CaO-doped ZrO2 solid solution (ZrO2-7.7 mol% Y2O3-4.4 mol% CaO); (ii) a mixture of ZrO2 and CaZrO3 having a crystal-orientation relationship; (iii) an amorphous phase containing elements of calcium, phosphorus, zirconium, and yttrium; and (iv) a remaining CaO-poor HA matrix (Ca10-xw(HPO4).(PO4)(6)-.(OH)(2-x:); = 0.06). Rationales for the greatly decreased impurity phases of CaO and Ca4P2O9 found in the composite coating are discussed.