Abstract: The ultrastructure of PAN-based preoxidized fibers prepared by a radio frequency plasma was investigated by scanning tunneling microscopy, and micrometer-, nanometer- and atom scale surface structural units in the preoxidized fibers were qualitatively defined. At the micrometer scale, microfibrils with different diameters and in strip-like form are aligned parallel to the fiber axis, and these microfibrils intersect each other. At the nanometer scale, the microfibrils are seen to be composed of closely packed and intertwined smaller fibrils. By making the scanning area smaller, it can be seen that the smaller fibrils are formed by chains of primary microcrystallites on the atomic scale, the carbon atoms on the surface of the preoxidized PAN fibers tend to be aligned but this is irregular compared with highly oriented pyrolytic graphite.