Nanofiber membranes of five different kinds of polymers have been prepared. The results of dielectric constant (k) measurements reveal that k-values of the polymers can be reduced dramatically by the electrospinning technique. The effects of the polymer types, the densities of the membranes, and the diameters of the fibers on the reducing of k-values have been investigated. It turns out that the unique microporous structures in the electrospun fiber films play an important role in decreasing the k-values of the polymer films. We anticipate that such a facile and general method may be applied to prepare ultra-low-dielectric-constant materials with potential application in the field of microelectronics.