Piezopolymers with high piezoelectric responses are highly desirable for a broad range of sensor, actuator and transducer applications. Defects modification concept applied on PVDF yields a high transverse piezoelectric response with both high piezoelectric d31 (d31=43.1pm/V) and electromechanical coupling k31 coefficients (k 31=0.187) in poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] 10wt% copolymers under quasi-static condition, much higher than these in the piezoelectric poly(vinylidene fluoride) (PVDF) and poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] copolymers. High frequency electromechanical resonance measurement was carried out under both LE mode and TE mode. Frequency dispersion of the piezoelectric response results in a smaller d31 (=20.5pm/V) at 5OkHz. Thickness resonance measurement gives kt=0.14 at 60MHz, which demonstrates it can be used as high frequency (>50MHz) ultrasonic transducers. Furthermore, the copolymers also display a higher d31 coefficient compared to the d33 coefficient, which seems to be unusual compared with most other piezopolymers. The experimental data suggests that the origin of the unusual piezoelectric response in these P(VDF-HFP) copolymers originates from a reversible change between a poled a-like structure and ß-like structure.