The purpose of this study was to examine or characterize the surface layer of a calcium phosphate ceramic with a gradual compositional change from alpha-tricalcium phosphate (alpha-TCP) on the surface to hydroxyapatite (HAP) on the inside. The surface of a dense HAP ceramic was acid-treated for 1 hour with orthophosphoric acid (H3PO4) solutions of several concentrations (0.5, 1.0 and 5.0 mol/L) or a buffered solution (pH 4.0) consisting of phosphate solutions. After acid treatment, specimens were heat-treated at 1,250 degrees C for 1 hour. X-ray photoelectron spectroscopy revealed that the compositional gradient layer could be modified on the surface of the HAP ceramic with all acid and heat treatments, and that 5.0 mol/L H3PO4 solution and heat treatments had a maximal thickness of approximately 2 microm for the surface-modified layer. It was confirmed that the outermost layer of HAP ceramics modified with the treatments, except 5.0 mol/L H3PO4 solution, showed a compound such alpha-TCP.