Thermal conductivity of carbon fibers (CFs) is an important property because CFs are used as heat dissipation fillers in composites for aerospace and electronics applications. However, evaluating thermal conductivity of a single filament of CFs is an arduous task due to dimensional issue of specimens and limitations of conventional measurement system. Therefore, we suggest an opto-thermal technique using Raman spectroscopy to measure thermal conductivity of commercial polyacrylonitrile based CFs (T300, T700SC and T800 H). The opto-thermal technique used that G band from Raman spectroscopy of carbon materials is shifted depending on temperature. For verifying an accuracy of the technique, the laser absorbance of CFs were estimated, and the thermal conductivity was measured depending on the length of CF. The measured data were reflected in the thermal conductivity calculation formula. It was demonstrated that the method provides more reasonable thermal conductivity values compare to a conventional Angstrom method. In addition, this simple technique confirmed that graphitic structure of CFs played a critical role in their thermal conductivity.