Stretchable and highly conductive films made of poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT/PSS) were prepared by casting the PEDOT/PSS aqueous dispersion as colloidal gel particles containing xylitol and subsequent heating. The electrical conductivity and elongation at break of the PEDOT/PSS film containing 50 wt% of xylitol significantly increased from 0.46 S/cm to 114 S/cm and from 4% to 33%, respectively, by heating at 140 °C in air for 1 h. It was found that the xylitol had two functions as (i) plasticizer preventing hydrogen bonding and (ii) secondary dopant increasing the mobility of charge carriers between the colloidal particles.