In the present work, the torsional modulus and internal friction of different types of polyacrylonitrile (PAN)- and pitch-based carbon fibers were measured with the torsional pendulum method to evaluate the relationship between these properties and the microstructure of carbon fibers. For easier and more accurate measurement, an aluminum disk-type pendulum was used. The measured torsional moduli were comparable to the previously reported values, which indicates the validity of the proposed method. The experimental results and discussion based on Eshelby's solution and Mori-Tanaka's mean stress method revealed that the torsional modulus should have a significant relation to the volume fraction of the crystalline region in the fibers and amorphous modulus. The results imply that fibers with a smaller crystalline region and higher amorphous modulus should show a higher torsional modulus. The internal friction of the fibers increased with the torsional modulus. This suggests that the internal friction may also be related to the size of the crystalline region and that the amorphous region has greater internal friction than the crystalline region.