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The carbonization behavior of polyoxadiazole (POD) fibers was investigated in comparison with that of films by using wide- and small-angle X-ray scattering, scanning electron microscopy and measurements of density, mass, volume and dimensional changes. On heat-treating POD fibers and films at a degradation temperature of about 500 °C, small-angle X-ray scattering from a long-period structure appeared and disappeared again, which indicated inhomogeneous progress of the scission of POD molecules. This accounted for the ability of this polymer to keep the macroscopic precursor geometry during carbonization in spite of yielding a graphitizing carbon. During the volume reduction process, the nuclei of the carbon layer stacks formed immediately after degradation of POD were piled up parallel to the material surface. This led to autonomous increase in the preferred orientation of the carbon layer stacks at higher temperatures. When the interlayer spacing decreased below 0.340 nm, the carbon layer stacks assumed three-dimensional regularity. The degree of the stacking regularity was higher for the films than the fibers. The macroscopic precursor geometry imposed a stronger restriction on the growth of the carbon layer stacks in the fibers than in the films.