Polydiene polymers(e.g. polyisoprene, polybutadiene) were doped with electron acceptor(e.g. iodine) and the changes of conductivity in doped polydienes were studied as a function of doping time and weight gains of dopant. After doping, the conductivities of doped polydienes were improved by 5∼9 orders of magnitude compared to those of undoped ones and represented the maximum value at ca. 0.65 mole of iodine per 1 mole of polymer unit UV/Vis. spectrum at about 260nm and 322nm indicated that the complex was formed between polydienes and iodine and polyiodide counter anions were created, respectively. In the FT-IR spectra, there is a red shift of -C=C- stretching peak from 1665cm^-1 to 1590cm^-1. The change in the -C=C- stretch indicated a weakening of the double bond. In the reaction of complex formation, the equilibrium constant(K) calculated by using Benesi-Hildebrand polt with optical density of the complex and concentration of polymer and iodine. Values of K were about 40 and 31 in the case of polyisoprene and polybutadiene, respectively. This result seemed to relate to the differences of electrical properties of polyisoprene and polybutadiene which were doped by iodine under same conditions.