For this study, we investigated the effects of reactive gases (oxygen, nitrogen, and argon) on the shear behavior and fracture toughness of HDPE/steel joints by treating high-density polyethylene (HDPE) with plasma using a microwave method. We also investigated the effect of plasma treatment on the physical and chemical changes on the surface of HDPE. HDPE/steel joints were fabricated using a secondary bonding process. The results showed that the shear strength and fracture toughness of HDPE/steel joints treated with different reactive gases were ordered as follows, oxygen > nitrogen > argon. Specifically, the shear strength and fracture toughness of oxygen plasma-treated HDPE/steel joints were approximately 7600% and 2400% greater, respectively, than that of untreated HDPE/steel joints. The improvements in shear strength and fracture toughness are attributed to increase in surface roughness and the creation of carbonyl functional groups on the HDPE surface via plasma treatment. (C) 2009 Elsevier B. V. All rights reserved.