Activated carbon fiber (ACF) is a material that has attracted significant attention because ACFs derived from several fiber sources have been adapted as filter materials. Here, we described successful ACF preparation via single- and two-step thermal treatments of rayon. Methylene blue adsorption abilities of the resulting ACFs were evaluated. Results indicated that ACFs derived from rayon which is prepared by the two-step thermal treatment demonstrate adsorption ability. Moreover, the average gray-scale intensities (GIs) of the carbonized samples were obtained from images captured with a common digital camera, and the results exhibited meaningful correlations for both yield and absorption ability with GI. Correlation between GI and absorption ability suggests the potential for a simple, nondestructive method of evaluating the absorption ability of ACFs. Results obtained for rayon is easily transferred to other fiber products or industrial waste materials generated during the manufacture of textiles that offers potential energy savings.