A Plasma Needle using a dielectric barrier discharge reactor at atmospheric pressure with a funnel-shaped nozzle was designed and realized. The preliminary characteristics of the plume and applications in the sterilization of Streptococcus mutans （S.mutans）, which is the most important microorganism causing dental caries, were presented in this paper. The reactor produced a cold plasma plume with temperature not higher than 315?K at 28?W. The effect of oxygen, which was injected downstream in the plasma afterglow region through a steel tube, was studied using its spectrum. The results show that addition of 26 mL·min-1 O2 led to the transport of atomic O further without significant effects on the jet length. The experiments show the best parameters are 15?W power, 01?m3·h-1 He with 25% O2 addition and separation of needle-to-sample 12?mm. It is found that ultraviolet rays, O，OH and He radicals can reach and act with S.mutans. To find critical radicals which are related to sterilization, we compareed the probe spectrum at atmosphereic pressure with that in agar.