Polyethylene terephthalate (PET) fibers have been increasingly used in textile industries for a variety of applications ranging from filtration, composites, tissue engineering and electronic textiles. The surface properties of these polymer fibers are of importance in various applications. The surface properties of PET fibers can be modified by different techniques. In this study, PET fibers were treated in oxygen plasma for improving surface wettability. The effects of plasma treatment on dynamic wetting behavior were characterized using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS) and dynamic contact angle measurements. The plasma treatment roughened the fiber surface revealed by atomic force microscopy (AFM). The introduction of functional groups was detected by XPS. The roughened and funtionalized surface resulted in the change in advancing and receding contact angles. Both advancing and receding contact angles were significantly reduced, but the contact angle hysteresis was increased after plasma treatment.