The thermal phase transitions of HCO2Cs and HCO2Rb were found to be reversible, first-order transitions. Through the transitions, the orthorhombic phase(Pbcm) of HCO2Cs transformed to the cubic phase (P3m3), and the orthorhombic phase (Pca21) of HCO2Rb transformed to the monoclinic phase (P21/m). From the viewpoint of the orientation of formate ion, the thermal phase transitions of HCO2Rb and HCO2Cs could be classified as order-order and order-disorder transitions, respectively. The Delta H-tr and Delta S-tr values for the transition of HCO2Cs were lager than those of HCO2Rb, which was predictable from the difference of the orientational changes of formate ions in both the transitions. When compared with the space around the formate ion for the phase after transitions of both the formates, the space for HCO2Cs is larger than that for HCO2Rb, and the formate ion of HCO2Cs could move more easily than that of HCO2Rb. This finding appears to reflect the difference of ionic radii of Rb+ and Cs+ ions.