Clay from tidal flat sediments showed an efficient catalytic activity in the decomposition of PCE by ozonation where the thermal treatment of clay enhanced the catalytic activity. No changes in XRD patterns of clays by the thermal treatment were observed, while EPR spectra were significantly changed. The intensity of the EPR signal that arises from natural radiation defect was decreased with increasing thermal treatment temperature, which enhanced the PCE removal efficiency. Contrary to ozonation along, superoxide radicals were detected from ozonation in the presence of clay. This suggests that the superoxide radicals further increase PCE decomposition.