The alpha-alpha' phase transition of tricalcium phosphate (TCP) has been investigated using high-temperature neutron powder diffraction and the Rietveld method. This transition occurs between 1458 and 1508 C. Hysteresis is not observed between heating and cooling within the observed temperature interval of 25 C. Structural refinement of the alpha'-TCP was successfully performed by the trigonal structure with space group P (3) over barm. The temperature dependence of unit-cell parameters of the TCP was determined around the a-a' phase transition point for the first time. At the a-a' phase transition point, the unit-cell parameters a, c, gamma angle, and unit-cell volume discontinuously changed, indicating that the transition is of first order. (C) 2003 Elsevier Science B.V. All rights reserved.