A novel approach for evaluating the torsional performance of single carbon fiber filament was established based on a self-designed apparatus. The number of torsion turns and twist angle at fracture from the direct torsion failure test were defined to characterize the torsion-bearing capacity of single carbon fiber. The fiber fracture strain under torsion stress was calculated based on a single fiber torsional deformation model and was compared with the fiber elongation in tension, which showed that the toughness of carbon fiber under torsion shear stress was inferior to its resistance to tensile stress. In addition, the resistance of carbon fiber to torsion shear stress was changed after changing the fiber surface structure by removing the sizing agent or coating the fiber surface with epoxy resin. The established method will enrich the existing research work on mechanical properties of carbon fiber as well as those for different types of fibers.