C-Ru-RuO2 nano-composite films were prepared by plasma-enhanced chemical vapor deposition and their microstructure and electrode properties were investigated. Ru-C nano-composite films consisted of Ru nano-particles of 3 nm in diameter and an amorphous C matrix. With increasing oxygen gas flow rate (FRO2), the volume fraction of C decreased from 0.91 to 0 and Ru nano-particle size increased from 2.5 to 4.5 nm. At high FRO2, the film consisted on the fibrous RuO2 and Ru-C nano-composite layer. Ru-C nano-composite containing 91 vol% C showed the highest interfacial electrical conductivity below 673 K, and Ru-C/RuO2 composite containing 0 – 5 vol% C showed the highest interfacial electrical conductivity at 873 K. Electro-motive-force (EMF) values of an oxygen concentration cell constructed from a YSZ electrolyte and Ru-C or Ru-C/RuO2 composite electrodes responded to the change of oxygen gas partial pressure at more than 473 K. The response time of the concentration cell with Ru-C nano-composite electrodes at 573 K was less than 10 s, and that with Ru-C/RuO2 composite electrodes was about 300 s.