A dc biasing method, developed in this work, has been investigated for the control of plasma space potentials and the chemical vapor deposition of nanocrystalline diamond (NCD) films in a planar surface-wave excited plasma at gas pressures below 100 mTorr. A negative dc voltage was applied to a specially shaped thin metal plate attached below the upper dielectric window with respect to the grounded substrate and discharge chamber, instead of the conventional positive substrate dc biasing method. Plasma parameters were measured using a single-probe and deposited films were evaluated by scanning electron microscopy, atomic force microscopy, and Raman spectroscopy. The application of the dc bias voltage (0 to -150 V) enabled the net dc bias current (-0.46–+0.6 A) to be varied and plasma space potentials to be decreased over a wide range (34–7 V) in the bulk region, resulting in the control of the bombarding ion energy on the grounded substrate. The vertical plasma parameter profiles showed the spatial difference in electron temperature between the local surface-wave region (∼10 eV) near the upper dielectric window and the bulk region (below 3 eV). It was found that the spatial difference in electron temperature permits the control of net currents and plasma space potentials in the dc biasing method. NCD films were deposited with smooth surfaces (rms=12.4 nm), a deposition rate of about 63 nm/h, and a continuous surface coverage on Si substrates maintained at a temperature of about 650 °C for hydrogen-based CO–H2 plasmas by biasing with -70 V to the metal plate.