An effective surface activation is crucial for high-speed roll-to-roll coating of functional films for printed electronics applications. In this article, we report a study of surface treatment of three types of poly(ethylene terephthalate) (PET) films by an argon/oxygen atmospheric pressure plasma and an ambient air atmospheric pressure plasma to obtain the required wettability for subsequent slot die coating of transparent conductive polymer layer using a poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) ink. Prior to plasma treatment, the PET surfaces, which differ in manufacturing process of their preparation, were characterized by X-ray photoelectron spectroscopy. The surface changes after the plasma treatments were characterized by water contact angle measurement and atomic force microscopy. We found that the water contact angles of the three types of untreated PET surfaces were 80.9 degrees, 75.9 degrees, and 66.3 degrees, respectively, and the water contact angles after argon/oxygen plasma treatment at treatment speed of 1mmin(-1) decreased to 36.2 degrees, 31.9 degrees, and 40.9 degrees, respectively. These conditions were stable from 1 up to 4 days, which are longer than reported values of 15-60min and sufficient for roll-to-roll coating processes.