This letter reports on the generation and characteristics of a large-scale dielectric barrier discharge plasma jet at atmospheric pressure. With appropriate parameters, diffuse plasma with a 50×5 mm2 cross-sectional area is obtained. The characteristics of the discharges are diag- nosed by using electrical and optical methods. In addition to being generated in helium, plasma is also generated in a mixed gas of helium and oxygen. The oxygen atomic radiant intensity （3p5P→ 3s5S, 3p3P→3s3S transition） is not proportional to the proportion of oxygen in the gas mixture, as shown by the experimental results.
