In this paper, water vapor up to 41% is added to an argon dielectric barrier discharge at medium pressure to profoundly investigate the effect of water vapor addition on the surface modification of polyethylene (PE). Contact angle measurements show that the wettability of PE significantly increases after a pure argon plasma treatment. However, the addition of water vapor to the argon feeding gas can give an extra 30% decrease in water contact angle. This extra decrease is due to the incorporation of additional oxygen containing groups such as CO, CO, OCO, and OCO. AFM results reveal that the surface roughness of PE only increases for plasma treatments with no or low water vapor concentrations. All this indicates that a water vapor based plasma can be an excellent tool for the surface activation of PE.