The ferroelectric phase transition was investigated in the VDF/TrFE copolymer films before and after irradiations by means of dielectric, X-ray diffraction, IR and gel content measurements. The diffuse phase transition was explained in terms of the Ginzburg-Landau theory, in which the domain boundary effect was taken into account. On irradiation, the peak temperature on the ε'-Tcurve shifted to lower temperatures, but did not decrease below 40°C even irradiated with more than 200 Mrad. On the other hand, the result of X-ray diffraction measurement exhibited a radiation-induced phase transition at room temperature after irradiation with 120 Mrad. This discrepancy may be attributed to the phase coexistence over a wide temperature range.