Dynamic tensile moduli of polyethylene-nickel-coated carbon fiber (NiCF) composites with 10 and 4 vol % NiCF contents under electrical field were measured by a homemade instrument in the frequency range of 100-0.01 Hz. The drastic descent of the storage modulus of the composite with 10 vol % was verified in lower frequency range with elevating surface temperature (T-s) by self-heating (Joule heat). The composite was cut when T, was beyond 108 degrees C. On the other hand, the measurement of the composite with 4 vol % beyond 88 degrees C was impossible, since T-s did not elevate because of the disruption of current networks. Incidentally, the dynamic tensile moduli by external heating could be measured up to 130 and 115 degrees C for 10 and 4 vol 96, respectively, but the two composites could be elongated beyond the above temperatures. Such different properties were analyzed in terms of crystal dispersions, electrical treeing, and thermal fluctuation-induced tunneling effect.