The diamagnetic susceptibilities of a polyacrylonitryle-based carbon fiber (PAN-CF) and benzene derived carbon fiber (BDF) stepwise heat-treated up to 3000°C were measured between liquid-nitrogen and room temperatures. Total susceptibility (χT) vs reciprocal temperature (1/T) plots for PAN-CF specimens heat-treated beyond 1300°C are entirely in accordance with McClure-Hickman's theory dealing with the transverse size effect on the orbital diamagnetism of π electrons in folded graphitic ribbons, while those of BDF are not for the specimens heat-treated above 2200°C. As the heat-treatment temperature is increased beyond 2200°C, the structure of BDF gradually becomes three-dimensional; finally, χT of a 3000°C-treated sample can be explained by a theory of Sharma, Johnson and McClure for singlecrystal graphite.