Fracture behavior and fracture toughness of pitch-based carbon fiber were studied using the fiber specimens into which an artificial mode I type straight-fronted edge notch was introduced by a focused-ion (Ga+)-beam. Two fiber samples were used for experiment. One was the as-supplied pitch-based fiber (GRANOC XN-35((R)), Nippon Graphite Fiber Corporation) and another was the same fiber heat-treated at 3273K for 3.6x10(3) s in argon atmosphere. The heat-treatment raised the Young's modulus from 370 to 840 GPa. The fracture toughness value of the as-supplied fiber was similar to 1.4MPam(1/2) and the apparent fracture toughness value of the heat-treated fiber was similar to 4.2MPam(1/2), which were estimated from the measured values of the strength (sigma(f)), notch depth (a), and fiber diameter (D) and correction factor. The increase in the apparent fracture toughness value due to the heat-treatment was attributed to the enhanced c-plane array and its cleavage fracture in longitudinal direction ahead of the notch tip. The analysis of the fracture strength - notch depth relation showed that the fracture of the as-supplied fiber obeys the fracture mechanical criterion and the fracture of the heat-treated fiber obeys the net stress criterion.