The aim of this study was to evaluate the effect of oxygen plasma treatment on the surface roughness, morphology, chemical surface structure, crystallinity and tensile properties of wool fiber yarn. The wool fibers were treated with oxygen plasma at a different treatment time. The morphological surface characterization of wool fibers was realized at sub-micro scale by means of high-resolution scandisk confocal microscopy and scanning electron microscopy. The chemical structure of untreated and plasma treated wool fiber surface was analyzed by Attenuated Total Reflectance Fourier Transform Infrared spectrometry. In addition, the percentage of crystallinity and the size of the crystals were investigated using an X-ray diffractometer. The results showed that oxygen plasma treatment leads to the removal of surface lipids and oxidizes the cysteine in the exocuticle and increases the surface roughness.