Normal ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymer have been converted into relaxor ferroelectrics by introducing appropriate amount of defects into the original polar crystallites, which is achieved either by high-energy electron irradiation or by copolymerizing with a third bulky monomer 1,1-chlorofluoroethylene (CFE). These modified P(VDF-TrFE)s exhibit typical relaxor ferroelectric behavior such as giant electromechanical response, the absence of ferroelectric-paraelectric transition, slim polarization loop, and a frequency-dependent strong dispersion in dielectric spectra.