Ultra-high molecular weight polyethylene (UHMWPE) fibers were subjected to oxygen plasma treatment in order to improve interfacial adhesion. The treated fibers were characterized by contact angle analysis, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), atomic force microscopy (AFM), and mercury porosimetry. The surface free energy, O 1s/C 1s ratio, and surface area increased dramatically with 1 min treatment. However, as the treatment time increased further, these parameters either increased slowly at 30, 60, and 100 W, or decreased at 150 W. The increased surface free energy is attributed to the polar component, while the increased O 1s/C 1s ratio is explained by the oxygen-containing moieties introduced by the plasma treatment. The oxygen plasma treatment also roughened the initially smooth surface of the UHMWPE fibers by forming micro-pores and thus increased the surface area. The interfacial shear strength of UHMWPE fibers to vinylester resin was measured by micro-droplet tests and exhibited an increasing trend, believed to result from the increased surface area, the surface free energy and the oxygen-containing moieties due to the plasma treatment.