Dielectric barrier discharge(DBD) cold plasma at atmospheric pressure was used for preparation of copper nanoparticles by reduction of copper oxide(CuO). Power X-ray di?raction(XRD) was used to characterize the structure of the copper oxide samples treated by DBD plasma. Influences of H2 content and the treating time on the reduction of copper oxide by DBD plasma were investigated. The results show that the reduction ratio of copper oxide was increased initially and then decreased with increasing H2 content, and the highest reduction ratio was achieved at 20% H2 content. Moreover, the copper oxide samples were gradually reduced by DBD plasma into copper nanoparticles with the increase in treating time. However, the average reduction rate was decreased as a result of the di?usion of the active hydrogen species. Optical emission spectra(OES) were observed during the reduction of the copper oxide samples by DBD plasma, and the reduction mechanism was explored accordingly. Instead of high-energy electrons, atomic hydrogen(H) radicals, and the heating e?ect, excited-state hydrogen molecules are suspected to be one kind of important reducing agents. Atmospheric-pressure DBD cold plasma is proved to be an efficient method for preparing copper nanoparticles. Dielectric barrier discharge (DBD) cold plasma at atmospheric pressure was used for preparation of copper nanoparticles by reduction of copper oxide (CuO). Power X-ray di? Raction (XRD) was used to characterize the structure of the copper oxide samples treated by DBD plasma. Influences of H2 content and the treating time on the reduction of copper oxide by DBD plasma were investigated. The results show that the reduction ratio of copper oxide was increased initially and then decreased with increasing H2 content, and the highest reduction ratio was achieved at 20% H2 content. Moreover, the copper oxide samples were gradually reduced by DBD plasma into copper nanoparticles with the increase in treating time. However, the average reduction rate was decreased as decreased as a result of the di? usion of the active hydrogen species . Optical emission spectra (OES) were observed during the reduction of the copper oxide samples by DBD plasma, and the reduction mechanism was explored accordingly. Instead of high- energy electrons, atomic hydrogen (H) radicals, and heating e? ect, excited-state hydrogen molecules are suspected to be one kind of heavy reducing agents. Atmospheric-pressure DBD cold plasma is proved to be an efficient method for preparing copper nanoparticles .