Ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymer [P(VDF-TrFE), with a content (mol %) ratio of 50/50 VDF/TrFE] were fabricated on silicon wafers covered with platinum by a spin-coating technique, ranging in thickness from 20 nm to 1 mum. The effect of thickness on dielectric properties and polarization behavior was investigated. A critical thickness was found to be about 0.1 mum. An abrupt drop of dielectric constant was observed, although there is no significant change in dielectric loss at this thickness. Square and symmetric hysteresis loops were obtained in films thicker than 0.1 mum. However, for films thinner than 0.1 mum, fewer square hysteresis loops were observed. SEM and X-ray results demonstrate that the effect of thickness on dielectric and ferroelectric properties could be explained by the changes of crystal structure in these films. In addition, the effects of irradiation on dielectric property and polarization response for ultrathin P(VDF-TrFE) films were also presented.