Ruthenium-Carbon (Ru-C) nano-composite films were prepared by microwave-induced plasma-enhanced chemical vapor deposition (PECVD) and the effects of deposition conditions on the microstructure and electrical properties were investigated. The films consisted of agglomerated grains of 10 to 20 nm in diameter, in which Ru particles of 2.5 to 3.5 nm in diameter were dispersed in an amorphous C matrix. The C contents of the films ranged from 86 to 94 vol%. The electrical properties of Ru-C nano-composite films as a catalytic electrode for an yttria-stabilized zirconia (YSZ) solid electrolyte were evaluated by AC impedance spectroscopy. The electrical conductivity at the Ru-C/YSZ interface (σi) was 0.2 × 10-3 Sm-1 at 500 K and increased with increasing temperature. The activation energy of the σi was 70 kJ/mol. Electro-motive-force (EMF) values of an oxygen concentration cell constructed from YSZ electrolyte and Ru-C nano-composite electrodes responded to the change of oxygen partial pressure above 473 K. The response time of the concentration cell was less than 10 s above 573 K.