In this study, a high-density polyethylene (HDPE, 5-mm-thick, 0.95g/cm³) surface was treated using two types of cold atmospheric pressure Ar plasma jets (CAPPLAT plasma and APC plasma). In particular, the effects of the additive gas (N₂ or O₂) on the HDPE surface treatment were investigated and compared in details. Using the two Ar plasma jets, a large number of O-containing functional groups were introduced onto HDPE surface during the plasma treatment. However, the effects of the additive gas on the HDPE surface treatment are quite different from each other. In case of CAPPLAT plasma treatment, the water contact angle (WCA) value of treated HDPE surface increased slightly when trace of the additive gas was added into CAPPLAT plasma. However, in case of APC plasma treatment, the WCA value of treated HDPE surface decreased significantly when the same amount of the additive gas was added.