We demonstrate an widely electrically tunable long-period fiber grating with ultrathin first cladding and ferroelectric relaxor poly(vinylidene fluoride -trifluoroethylene - chlorofluoroethylene) terpolymer as the second cladding. Large Kerr effect is found in the terpolymer where a refractive index change of -2.6% can be induced under an electric field of 80MV/m. Simulations and experiments show that electrodes and the index matching between terpolymer and fiber have significant effect on the tuning range. An 18nm resonant wavelength shift is achieved by the terpolymer when electric field of 50MV/m is applied. On the other hand, over 100nm shift is observed by index matched terpolymer/PMMA blend as the temperature changes from 25 °C to 100 °C (temperature tuning). To realize this index matching condition, ZnS/terpolymer nanocomposite was developed which allowed the index of the composite to be varied over a large range while maintaining large electro-optical response. A simulation result predicts that large electrical tuning of the resonance band can be realized by the the index matched nanocomposite.