By following the progression of continuous carbonization (300–1250°C) of polyacrylonitrile (PAN)-based oxidized fibers, variations in tensile mechanical properties and morphological structure are reported in detail along the carbonization line. The tensile strength and Young's modulus of the fibers generally increase throughout the carbonization stage. Meanwhile, the fiber diameter displays a significant decrease. The preferred orientation of carbon layer planes is observed to increase remarkably for temperatures over 400°C. In a similar manner, the stacking size increases significantly but reaches saturation around 600°C, a changing point corresponding to that observed for the variation of the Young's modulus. However, beyond about 1200°C, the stacking size again displays a marked increase. Results thus obtained are interpreted in relation to each other. Possible implications are also discussed. © 1994 John Wiley & Sons, Inc.