Study of an amorphous phase in plasma-sprayed hydroxyapatite (HA) coatings is important owing to its unique characteristics and nonnegligible amount of the amorphous phase compared to crystalline HA. However, little is known about the component parts of an amorphous phase. It is known that amorphous phase usually appears as the diffusion maximum (D max ) in X-ray diffraction (XRD) patterns. Analyzing D max , including the position (P max ) and area of D max , we can indicate the component parts of an amorphous phase and their transitions. In this study, the variation of D max in XRD patterns of the coatings during plasma spraying, in postheating, and in dissolving in vitro was studied with the aid of XRD. It was found that component parts of the amorphous phase in the coating varied with increasing thickness, consisting of two parts represented by D max1 , located between 29.4 and 29.8 degrees (2), and D max2 , located between 31.0 and 31.4 degrees (2). It was concluded that D max3 , located between 32.0 and 32.4 degrees (2), should be referred to as nanocrystals of HA. In addition, the particle size of the starting powder may affect the component parts of the amorphous phase in the coating in addition to thickness. With vacuum heating (650ยฐC) and water vapor treatment at a low temperature (125ยฐC) in a satu-rated vaporic atmosphere, transition of the amorphous components was not as efficient as that at 490ยฐC with water vapor. The reason might be that the amorphous-to-crystalline HA conversion is dependent on both temperature and water vapor pressure. It was found that amorphous components were transformed completely into crystalline HA after heating at 490ยฐC with a partial water vapor pressure of 0.01 MPa for 2 h. It was concluded that the unstable amorphous components (D max1 , D max2 ) converted into more stable nanocrystals of HA (D max3 ). Degradation in vitro showed that D max3 was more stable than D max1 and D max2 . It was concluded that nucleation of apatite in vitro should be attributed to nanocrystals of HA (D max3 ) except for the amorphous components. It is recommended that the optimal phasic contents of the plasma-sprayed HA coating be mainly composed of crystalline HA and nanocrystals of HA (D max3 ) in terms of the stability and biocompatibility of the coating.