High performance rigid-rod polymeric materials provide potential applications as reinforcement fibers for advanced composites. In this study, we investigated the surface modification of poly( 1,4-phenylene-cis-benzobisoxazole) (PBO) fiber by oxygen plasma treatments. Kevlar and carbon fibers were also examined for comparison. The corresponding changes in the surface free energy components were carefully evaluated using a Cahn dynamic contact angle analysis system. The results showed that the total surface free energy (gamma) of PBO fiber was increased from 43.3 to 61.1 mJ m(-2) (by 41%) using 70 W oxygen plasma treatment for 5 min. The polar component of the surface free energy increased much more readily than the dispersive component. In addition, the untreated PBO fiber had a tensile strength of 5.72 GPa while the treated PBO fiber had an average of 5.55 GPa. This represented a small reduction of only 3%. The effect of oxygen plasma treatment on the composite's interfacial adhesion property was carried out using microbond fiber pull-out test. The interfacial shear strength increased from 34.7 MPa for the untreated PBO fiber system to 44.7 MPa, after the oxgen plasma treatment for 5 min. It has been suggested that oxygen plasma is an effective process for rigid-rod PBO fiber. (C) 2004 Elsevier B.V. All rights reserved.