The molecular motion and structure transitions of γ-irradiated trioxane and its polymer have been studied by the dielectric measurement. It was found that there was a change of microstructure (crystal lattice defect) in trioxane between 45℃ and 55℃. Release from defect and recombination of active species (R,+,) produced by radiation took place with rise of the temperature of the system. Since the polarity and the molecule motion lessened after postpolymerization of irradiated trioxane, the dielectric loss tangent (tg,δ,) was decreased. The transition from trioxane to polyoxymethylene has been observed from the dielectric spectra. The structure change of polyoxymethylene resulted from radiation degradation has been discussed by means of the dielectric spectrum.