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A nonequilibrium Ar/O2 plasma at atmospheric pressure was generated between two parallel square electrodes by a 13.56-MHz power supplier. Experimental results demonstrate that the Ar/O2 plasma more effectively kills Staphylococcus aureus and Escherichia coli than the Ar plasma. When the O2 flow rate increases from 0 to 100 sccm step by step, the discharge voltage and the radio-frequency (RF) current always show sinusoidal waveforms, and the discharges maintain the α mode as well. In addition, the sterilization efficiency values of the RF-powered atmospheric pressure Ar/O2 plasma depend on the oxygen fraction in Ar gas. It is found that the sterilization efficiency of the Ar/O2 plasma is highest with 0.5 vol.% oxygen in argon gas, corresponding to the maximum production of O(3P) radicals. It is considered that the O(3P) radicals produced by the Ar/O2 plasma play a significant role in atmospheric plasma sterilization.