The phase transition of cesium formate observed at around 36 degrees C was found to be a reversible first-order transition by means of thermal analyses, The results of the Rietveld analysis of powder X-ray diffraction data showed that an orthorhombic phase (Pbcm, Z = 8; a = 4.785(1), b = 9.553(9), and c = 15.927(2) Angstrom) transformed to a cubic phase (Pm3m, Z = 1; a = 4.500(5) Angstrom). The large values of enthalpy and entropy change for the transition (Delta H-tr = 6.87 +/- 0.28 kJ mol(-1) and Delta S-tr = 22.2 +/- 0.9 J K-1 mol(-1)) were discussed on the basis of the change of the orientation freedom of the formate ion.