This paper discusses several structural, electrical and oxidation characteristics of co-sputtered Ta-Ru alloy films on oxidized Si-substrates. From X-ray examination, the Ta1Ru1 phase has formed and dominates in the compositions exceeding 54 at.% Ru content. The resistivity of the Ta-Ru thin films can reach a maximum of approximately 320 μΩ cm in the composition range between 35 and 54 at.% Ru. After thermal treatment in air (600 °C, 1 h), Ru-rich samples show a less increase in resistivity than Ta-rich ones. The observed preferential oxidation of Ta in the Ta-Ru samples can be further interpreted by thermodynamic calculations. The Ta-rich surface oxide is believed to be responsible for the passivating ability of the Ru atom toward oxidation at high temperatures. This results in the Ru of the metallic state though the oxidation of Ta occurs.