Abstract The polyacrylonitrile-based (PAN-based) carbon fibers with progressive amounts of the oxidation treatment have been investigated using a number of surface analytical methods. The chemical content and structure of oxidated fibers have been interrogated by X-ray photoelectron spectroscopy (XPS) and scanning electron microscope (SEM), then compared to the untreated ones. The value of oxygen content increased from 4.71% to 18.84%, meanwhile, the grooves on the surfaces of the carbon fibers were deepened gradually after treatments. The results of dynamic contact angle (DCA) measurements suggest that the increase in carbon fiber surface tension is mainly because of the polar component of surface tension. After treated with increasing amounts of oxidants, the monofilament tensile strength decreased less than 3.1%.