Trifluoroethylene addition and thermal treatment induced crystal phase transition in a series of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-co-TrFE)] containing varied TrFE molar ratio (6, 9, 12, and 20 mol %) prepared from the hydrogenation of poly(vinylidene fluoride-co-chlorotrifluoroethylene have been investigated by means of Fourier transform infrared spectral (FTIR), Xray diffraction (XRD), and differential scanning calorimetry (DSC) analyses. The comprehensive applications of the three techniques could distinguish alpha, beta and gamma phase of P(VDF-co-TrFE) very well. The multipeak fitting technique of DSC is successfully applied to calculate the percentage of different phases in the samples, which allows us to investigate the phase transition process of P(VDF-co-TrFE) precisely. It is found that the crystal phase of P(VDF-co-TrFE) films is turned from alpha + gamma phase (6 mol % TrFE) to alpha + gamma + beta phase (9 and 12 mol % TrFE) to beta phase (20 mol % TrFE) at high temperature, and from alpha + gamma phase (6 mol % TrFE) to gamma + beta phase (9 mol % TrFE) to beta phase (>12 mol % TrFE) at low fabricated temperature. Both the fabrication conditions and TrFE addition are responsible for the crystal phase transition of the hydrogenised P(VDF-co-TrFE). (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 3002-3008, 2013