Low density polyethylene (LDPE) and high density polyethylene (HDPE) were plasma-treated with N-2 and O-2 plasma. The wettability and polar component of surface free energy of plasma-treated polyethylene were investigated by contact angle measurement. The concentration of functional groups formed by plasma treatment such as hydroxyl and carbonyl groups was measured using attenuated total reflection Fourier transform infrared spectroscopy (ATR FTi.r.). The concentration of polar functional group increased rapidly with 5-10 s of plasma treating time and then very slowly after that. The adhesion strength of epoxy resin/plasma-treated polyethylene joints was examined by a 90 degrees peel test. The increase of the adhesion strength was similar to that of concentration of polar functional groups. The higher adhesion strength of epoxy resin/plasma-treated HDPE joints was observed than that of epoxy resin/plasma-treated LDPE joints since HDPE deformed more during the peel tests and had more polar functional groups on the surface.