Thermal stabilization parameters are the key factors in the production of carbon fibers. The combined effect of stepwise temperature, dwell time and stretching on the properties and structures of PAN fibers was studied by elemental analysis, bulk density determination, tension measurement, FTIR and WAXD. It is indicated that the whole process of thermal stabilization can be roughly divided into four stages by three temperatures of 220 degrees C, 240 degrees C and 270 degrees C, of which the increment rate of oxygen content, as well as the bulk density of fibers, is different, while the dwell time has no obvious effect on them until about 240 degrees C. Stretching plays a great role on the tension in fibers at early stabilization stage, while temperature contributes more at later stage. The stretching is also helpful to suppress the growth of crystallite below 220 degrees C, which has not been reported in published documents.