To investigate the effect of the gas composition of the plasma treatment on the surface modification of an atmospheric pressure plasma jet (APPJ), nylon 6 films were treated with APPJ with pure helium (He), He + 1% oxygen (O-2), and He + 2% O-2, respectively. Atomic force microscopy showed increased surface roughness, whereas X-ray photoelectron spectroscopy revealed increased oxygen contents after the plasma treatments. The plasma-treated samples had lower water contact angles and higher T-peel strengths than the control. The addition of a small amount of O-2 to the He plasma increased the effectiveness of the plasma treatment in the polymer surface modification in terms of surface roughness, surface oxygen content, etching rate, water contact angle, and bonding strength. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2201-2206, 2011