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In this paper, plasma immersion ion implantation (PIII) treatments of carbon fibers (CFs) are performed in order to induce modifications of chemical and physical properties of the CF surface aimed to improve the thermoplastic composite performance. The samples to be treated were immersed in nitrogen or air glow discharge plasma and pulsed at -3.0 kV for 2.0, 5.0, 10.0, and 15.0 min. After PIII processing, the specimens were characterized by atomic force microscopy (AFM), scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Surface morphology of as-received CFs exhibited some scratches aligned along the fibers due to the fiber manufacturing process. After both treatments, these features became deeper, and also, a number of small particles nonuniformly distributed on the fiber surface can be observed. These particles are product of CF surface sputtering during the PIII treatment, which removes the epoxy layer that covers as-received samples. AFM analyses of CF samples treated with nitrogen depicted a large increase of the surface roughness (Rrms value approximately six times higher than that of the untreated sample). The increase of the roughness was also observed for samples treated by air PIII. Raman spectra of all samples presented the characteristic D- and G-bands at approximately 1355 and 1582 cm-1, respectively. Analysis of the surface chemical composition provided by the XPS showed that nitrogen and oxygen are incorporated onto the surface. The polar radicals formed on the surface lead to increasing of the CF surface energy. Both the modification of surface roughness and the surface oxidation are expected to contribute for the enhancement of CF adhesion to the polymeric matrix used for composite manufacturing.