For the direct measurement of the fracture toughness of the carbon fiber, a new technique was proposed and examined its applicability. At first, machining condition of the notch was examined. The notch was introduced using focused ion beam (FIB). The ion beam can be electronically scanned to introduce a sharp notch on the carbon fiber. Notches with various notch width and length were introduced by changing beam and scanning conditions. Tensile tests on notched carbon fibers were carried out following the test method for carbon fiber monofilaments. Fractured specimens were successfully corrected without secondary damage using protection films. SEM observations revealed that a crack propagated from a notch-tip, and notch size was able to be determined successfully. Effect of notch root radius was also examined to investigate the validity of the fracture toughness obtained by this method.