Introduction: Since transparent conductive materials with indium tin oxide (ITO) involve some problems, non-ITO transparent conductive materials have been studied in these days. One of the promising candidates is a conductive polymer, poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonic acid) (PEDOT:PSS). PEDOT:PSS has not high conductivity enough to be replaced for ITO. Therefore, lots of second dopants with a conductivity enhancement ability have been studied. The authors have been studying the second dopant effect of dihydroxymethylphenol derivatives (DHMPs). Methods: The mixtures of PEDOT:PSS and DHMPs with several kinds of alkyl substituents and their isomers were prepared in iso-propyl alcohol/water mixture, and the mixtures were coated and dried on the glass slide. The surface resistivity of the dried coated layer was evaluated with the four-point probe device. UV spectra, zeta-potential and size of PEDOT:PSS/DHMPs dispersions and solids were measured with the UV-Vis spectrometry and the zeta-potential & size analyzer. In addition, the computer simulation method was used in order to examine our assumption about the conductivity enhancement mechanism. Results & Discussion: The study of the addition effect of DHMPs on the conductivity of PEDOT:PSS disclosed that, 1) the conductivity enhancement ability of DHMPs was dependent on their chemical structure of their substituent and isomer, 2) 4,6-dihydroxymethyl-2-methylphenol (46DHMP) had the highest conductivity enhancement ability among DHMPs, 3) a new shoulder peak appeared around the wave length of 360 nm in the UV spectra of DHMP/PEDOT:PSS solids, which were attributable to the electronic interaction between them to enhance possibly the conductivity of PEDOT:PSS. The action mechanism of DHMPs on the conductivity enhancement of PEDOT:PSS was proposed based on the above results, which was supported by the computer simulation.