We investigated the transition between the monoclinic (P2(1)/b) and hexagonal (P6(3)/m) phases of hydroxyapatite, Ca-5(PO4)(3)OH, both by X-ray diffraction (XRD) and differential scanning calorimeter (DSC) techniques. Peak intensities of XRD of the low-temperature monoclinic phase were almost unchanged up to 473 K but dropped abruptly to zero above 483 K. Endo- and exothermic DSC peaks and a specific heat anomaly associated with the transition were observed. The transition enthalpy and entropy were estimated to be 630 +/- 25 J/mol and 1.30 +/- 0.05 J/mol . K, respectively.