A large electromechanical coupling factor kt of about 0.3 has been found in vinylidenefluoride and trifluoroethylene copolymers containing 70–80 mol% VDF. These copolymers exhibit clear ferroelectric behavior. The strong piezoelectricity and distinctive ferroelectricity are attributable to high crystallinity, strong preferred orientation, and reduced conformational defects in crystallites, which are induced by annealing and successive or simultaneous poling. The polarization is inferred to grow through rotation of the polar axis by nπ/3 about the molecular axis (c-axis) under an external electric field above the coercive field.