Electrical conduction in polymers under a relatively low applied electric field is considered to be ionic and is affected strongly by the structural factors of the polymers. The following equation for the electrical conductivity &sgr; was derived in which free volumeVf, jump energyEj, and ionic dissociation energyWwere taken into consideration:&sgr;=&sgr;0exp− [&ggr; Vi*/Vf+(Ej+W/2&egr;)(kT)−1], where &sgr;ois a constant, &ggr; the numerical factor to correct the overlap of free volume,Vi*the critical volume required for transport of an ion, &egr; the dielectric constant,kBoltzmann's constant, andTthe absolute temperature. This equation describes well the conduction phenomena in polymethylmethacrylate, polystyrene, and an unsaturated polyester. Relationships between electrical conduction and free volume are discussed.