Abstract: Objective:?A?direct-current,?cold?atmospheric-pressure?air?plasma?microjet?(PMJ)?was?performed?to?inactivate?Staphylococcus?aureus?(S.?aureus)?and?Enterococcus?faecalis?(E.?faecalis)?in?air.?The?process?of?sterilization?and?morphology?of?bacteria?was?observed.?We?wish?to?know?the?possible?inactivation?mechanisms?of?PMJ?and?explore?a?potential?application?in?dental?and?other?temperature?sensitive?treatment.?Methods:?In?this?study,?we?employed?a?direct?current,?atmospheric?pressure,?cold?air?PMJ?to?inactivate?bacterias.?Scanning?electron?microscopy?was?employed?to?evaluate?the?morphology?of?S.?aureus?and?showed?rupture?of?cell?walls?after?the?plasma?treatment?and?Optical?emission?spectrum?(OES)?were?used?to?understand?the?possible?inactivation?mechanisms?of?PMJ.?Re-sults:?The?inactivation?rates?could?reach?100%?in?5?min.?When?the?distance?between?the?exit?nozzle?of?the?PMJ?device?and?Petri?dish?was?extended?from?1?cm?to?3?cm,?effective?inactivation?was?also?observed?with?a?similar?inactivation?curve.?Conclusion:?The?inactivation?of?bacteria?is?attributed?to?the?abundant?reactive?oxygen?and?ni-trogen?species,?as?well?as?ultroviolet?radiation?in?the?plasma.?Different?life?spans?and?defensibilities?of?these?killing?agents?may?hold?the?key?to?understanding?the?different?inactivation?curves?at?different?treatment?distances.