We have investigated highly conductive and transparent thin films of poly(3,4-ethylenedioxythiophene)/poly(4-styrene-sulfonate) (PEDOT/PSS) bar-coated on transparent plastic substrates such as polycarbonate (PC) and poly(ethylene terephthalate) (PET) sheets, in terms of sheet resistance (R-s), transparency (T), and surface roughness (R-a). Plasma treatment of the substrate and the addition of a surfactant and diethyl glycol (DEG) in the centrifuge-treated PEDOT/PSS dispersion are optimum coating conditions to obtain high-performance transparent PEDOT/PSS/PET sheets whose sheet resistance (R-s), transmittance at 550 nm (7), and surface roughness (R-a) were 383 Omega/sq, 75%, and 1.8 nm, respectively.