Thick (ca. 700 mu m) freestanding polycrystalline diamond films, 10 X 10 mm, were obtained using a high power (4 kW) microwave plasma-enhanced chemical vapor deposition system. Prior to fabricating the photoconductors, we treated the growth side (as-grown surface of the film) and the substrate side (the interfacial surface of the film detached from the Mo substrate) using H-2 and/or O-2 plasma. Using these films, we fabricated parallel-type ohmic-contacted diamond photoconductors. At as-grown states, the growth side showed noticeable photoconductivity, while the substrate side showed little photoconductivity. O-2 plasma treatment of these sides led to the extinction of the photoconductivity. After exposing these sides again to H-2 plasma, we observed not only distinct photoconductivity at the substrate side but also the reappearance of photoconductivity at the growth side. Based on these results, we suggest the primary factor for the photoconductivity of these films.