  The shape evolutions of the pear-shaped nuclei $^{224}$Ra and even-even
$^{144-154}$Ba with temperature are investigated by the finite-temperature
relativistic mean field theory with the treatment of pairing correlations by
the BCS approach. The free energy surfaces as well as the bulk properties
including deformations, pairing gaps, excitation energy, and specific heat for
the global minimum are studied. For $^{224}$Ra, three discontinuities found in
the specific heat curve indicate the pairing transition at temperature 0.4 MeV,
and two shape transitions at temperatures 0.9 and 1.0 MeV, namely one from
quadrupole-octupole deformed to quadrupole deformed, and the other from
quadrupole deformed to spherical. Furthermore, the gaps at $N=$136 and $Z=$88
are responsible for stabilizing the octupole-deformed global minimum at low
temperatures. Similar pairing transition at $T\sim$0.5 MeV and shape
transitions at $T$=0.5-2.2 MeV are found for even-even $^{144-154}$Ba. The
transition temperatures are roughly proportional to the corresponding
deformations at the ground states.
