The polyvinylidene fluoride trifluoroethylene P(VDF-TrFE) copolymer is polymorphic with two main phases called α (paraelectric) and β (ferroelectric). Effects of annealing, cooling, and poling on the phase transitions are studied in thin films. More precisely, the α-β structural transition is investigated in copolymer capacitors with flexible electrodes and substrate using in situ synchrotron X-ray diffraction. After annealing above the Curie transition, another phase called δ is put in evidence during cooling as α transforms into β. With combined effect of annealing and poling, a sharp phase transition is put in evidence from β to δ at 96 °C after stopping poling (E = 70 MV m−1) and a continuous phase transition occurs from δ to α in the range from 100 to 110 °C. The δ phase has high crystallinity with a domain size (normal to the film surface) of approximately 50 nm i.e. more than three times larger than for β. These phase transformations occur at relatively small electric field because of additional thermal effect and stress change for β and δ/α due to morphological changes. Permittivity and ferroelectric measurements performed on the copolymer capacitors allow confirming the phase transitions from β to δ and finally to α.