We introduce in this article oxygen plasma treatment as a convenient and effective method for the surface modification of Armos fibers. The effects of oxygen-plasma-treatment power on both the Armos fiber surface properties and Armos-fiber-reinforced poly(phthalazinone ether sulfone ketone) composite interfacial adhesion were investigated. The Armos fiber surface chemical composition, surface morphology and roughness, and surface wettability as a function of oxygen-plasma-treatment power were measured by X-ray photoelectron spectroscopy, scanning electronic microscopy, atomic force microscopy, and dynamic contact angle analysis. The results show that oxygen plasma treatment introduced a lot of reactive functional groups onto the fiber surface, changed the surface morphology, increased the surface roughness, and enhanced the surface wettability. Additionally, the effect of the oxygen-plasma-treatment power on the composite interfacial adhesion was measured by interlaminar shear strength with a short-beam bending test. Oxygen plasma treatment was an effective method for improving the composite interfacial properties by both chemical bonding and physical effects. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 2804-2811, 2011