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An atmospheric-pressure plasma jet generated in O2/Ar mixtures by specially designed equipment with two coaxial quartz tubes and double power electrodes has been investigated, and its effect on the cleaning of surface organic contaminations has been studied. The Q-V Lissajous figures are performed to evaluate the power consumed in the discharge and show no great modification in consumed power with the increase in the oxygen flow rate. From the results of the optical emission spectra, remarkably high oxygen radical concentration is obtained at a 1.5% addition of O2 to Ar and then decreases with the further increase in the O2/Ar mixing ratio. The effect of the surface cleaning by O2/Ar-based plasma is studied with respect to the changes in the contact angle. An addition of O2 to Ar decreases the contact angle, and the lowest contact angle is obtained at a 1.5% addition of O2 to Ar. However, further addition of oxygen does not show further improvement in the contact angle. From the results of quadrupole mass-spectrum analysis, we can identify the fragment molecules of CO and H2O in the emitted gases, which are produced by the decomposition of the surface organic contaminations during the cleaning process.