Flexible polymer thermoelectric devices based on poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and pyromellitic dianhydride‐oxydianiline polyimide polyimide (PI) were fabricated and investigated in this work. PI was selected as a substrate for PEDOT:PSS to secure from repeated bending cycles of flexible device. To enhance the interfacial adhesion between PEDOT:PSS and PI, oxygen plasma treatment was used on the surface of PI substrate. The effect of the surface treatment with oxygen plasma on the synthesized PI substrate was significant. The polar component of surface free energy of PI was increased from 2.8 to 31.8 mJ/m2. The power factor of PEDOT:PSS on the PI substrate was increased from 25.86 to 43.78 μW m−1 K−2. Also, as a result of 10 k times of bending test, the electrical performance consistency and the mechanical stability of the fabricated devices were confirmed. This verified fabricated flexible polymer thermoelectric devices based on PEDOT:PSS and PI are suitable for the various applications.