A series of high dielectric constant polymers have been developed with K from 10 to over 50. The high-K polymers have high dielectric breakdown strength above 700 V/μm and high energy density up to 27 J/cm3 in lab-scale small sample test. As thermoplastic semicrystalline polymers similar to polypropylene, these high-K polymers can be processed into thin capacitor film using melt-extrusion and biaxial orientation process and film thickness from 2 μm to 10 m has been achieved. Metallized prototype capacitors show promising performance with energy density above 5 J/cm3. Future development will target at 10 J/cm3 energy density in the packaged capacitors. On the other hand, the capability to produce ultrathin capacitor film enables the manufacturing of film capacitors with high capacitance density above 10 μF/cm3. Potential applications include pulsed power capacitors for external and implantable medical defibrillators, and other industrial applications.