NiOx films were deposited with radio-frequency sputtering at various O-2/Ar gas-flow ratios using a nickel-metal target, and pentacene thin-film transistors (TFTs) were made with NiOx as a source-drain (S/D) electrode. The oxidation state of Ni in the film was confirmed with X-ray photoemission spectroscopy to be 0, +2, and +3, and with an increase of the O-2/Ar flow ratio, the fraction of the higher oxidation state was increased. With an increase of oxygen in the film, the conductivity was increased, and the work function was also increased to 5.2 eV, close to the highest occupied molecular orbital of pentacene, which reduces the barrier height between S/D electrode and semiconductor. Pentacene TFTs with NiOx film deposited at Ar:O-2 = 0:50 sccm showed much better performance than pure Ni with a work function of 4.6 eV due to the reduction of the hole injection barrier at the S/D electrode-semiconductor interface. (C) 2008 The Electrochemical Society.