  Thermal inflation is an attractive solution to the cosmological moduli
problem. However, domain walls may be formed after thermal inflation and some
mechanisms are needed to eliminate the domain wall before it dominates the
Universe. We point out that gravitational waves produced by the dynamics of
domain walls may be observed by the pulsar timing experiments and future
space-borne gravitational wave detectors, which provides a probe into the
period of thermal inflation. We also show that the QCD instanton effect can
effectively eliminate the domain walls with producing observable amount of
gravitational waves.
