Structural evolution of polyacrylonitrile precursor fibers during preoxidation and carbonization were studied using SEM combined with XRD, FTIR, elemental analysis and density measurement. Crystallite structure of fibers has been completely changed through the process. Crystallite size of fibers firstly increases before 235 degrees C, and then decreases with temperature increasing, whereas crystallinity gradually decreases during preoxidation. The combination between fibrils during preoxidation firstly becomes looser, and then gets denser. Meanwhile fibrils at first grow thicker, followed by getting thinner. Homogeneous and granular fracture surface of the resultant carbon fiber is obtained. Fracture morphology during the preparation undergoes transformation from ductile fracture feature to brittle fracture feature, which corresponds to changes of tensile strength and elongation at break from 0.865 GPa to 3.51 GPa, from 9.8% to 1.7%, respectively.