Abstract: Microvoids, formed in spinning, oxidation and carbonization during the preparation of PAN-based carbon fibers, were investigated by two-dimensional small angle X-ray scattering. Results indicated that microvoids in PAN fibers had a preferred elongation and orientation along the fiber axis with an average length of 24.3 nm, diameter of 19.2 nm and aspect ratio of around 1.27. The size of the microvoids remained almost unchanged during oxidation, while their aspect ratio increased to 1.85, possibly due to a crystalline transformation from a linear structure in PAN fibers to a heatresistant ladder structure in oxidized fibers. However, the size of the microvoids decreased dramatically to about 3.56 nm in the long axis and 2.85 nm in the short axis during carbonization. The surface fractal dimension (Ds) increased in these processes from 2.42 to 2.88 and the most severe change occurred in low-temperature carbonization, indicating that surface roughness increased during processing, and carbonization was the most important process for such a transformation