Abstract: It is of significant importance to improve the dielectric property and the electric breakdown performance of polymer dielectric materials for their successful applications in the impulse capacitors as they are related with the large energy density and the high cycle efficiency, which are desirable for applications in various military and civil domains. As a type of polymer dielectric materials, the vinylidene fluoride-hexafluoropropylene copolymer (P(VDF-HFP)) enjoys excellent comprehensive properties. However, its dielectric property remains to be improved. In this paper, the P(VDF-HFP) films are first made through a solution-casting process, and then isothermally annealed under serially changing temperatures and times. The crystal form, the crystallinity and the β-phase content of the resulting films are characterized with the X-ray diffraction (XRD), the Fourier transformed infrared (FTIR) spectroscopy and the differential scanning calorimetry (DSC), respectively, and the dielectric properties of the films are assessed to see the effects of annealing. It is indicated that the relative fraction of the β-phase in the P(VDF-HFP) films can be effectively improved simply by annealing under adequate temperatures and times, with considerably enhanced dielectric performance. Upon being annealed at 120℃ for 12 h, the P(VDF-HFP) film is found to have the highest proportion of the β-phase up to 92.1%, with a high dielectric constant of 15.3 (100 Hz, 45% higher than the original film) and low dielectric loss of 0.019.