In this paper, we want to develop and characterize a round argon-based nonthermal atmospheric-pressure plasma jet (APPJ) with an application of sterilization. We measured the basic properties of APPJ by an alcohol thermometer, an optical emission spectrometer (OES), a high-voltage probe and a Rogowski coil. The plasma jet temperature at the tube exit is lower than 37°C, which will not induce damage to living tissue. The plasma absorbed power is about 2.7 W. The OES data indicate that this APPJ has a relatively high intensity of hydroxyl radicals (OH) in postdischarge region. Furthermore, we applied this device to inactivate the bacteria (E. coli and B. subtilis) and endospore (B. subtilis endospore). In addition, the trace amount of oxygen addition (0.04%) into working gas can not only enhance the OH radicals but also speed up the killing of bacteria and endospore. The results of D value and scanning electron microscope images show its great potential for the use of general sterilization in the future.