Fatigue tests of carbon fiber made from polyacrylonitrile (PAN) were performed to clear S-N properties and discuss the relationships between the fatigue fracture toughness and the pattern of fatigue fracture surface. The fatigue tests were carried out at a frequency of 10Hz with three stress ratios of 0.1, 0.5 and 0.7. It was found that S-N properties of carbon fibers clearly showed the fatigue behavior, depending on the stress ratio R. The fatigue lives of carbon fiber showed a large scatter. Mirror-mist-hackle patterns were clearly observed on the fracture surface of specimen tested under cyclic loading. As outside length on mirror region decreased, the form of the mirror region in each stress ratio changed from a semicircle to a half-ellipse. Relations between K_max or ΔK calculated from their area and fatigue life can be approximated in a straight line. Fatigue crack growth behavior is governed by two parameters such K_max and ΔK, and combined parameter K_max^(1-α)×ΔK^α is useful to describe fracture toughness at the different stress ratio.