Dependence of the tensile strength of resulting carbon fiber (CF) on the preparation technologies during preoxidation and carbonization were studied systematically by a series of pilot experiments. The proper preoxidation time, preoxidation temperature, and preoxidation stretching ratio are the base of preparing highquality CF. During precarbonization, the enhancement of the precarbonization temperature and the application of the precarbonization stretching are helpful to increase the tensile strength of CF, but the stretching ratio should be controlled carefully by on-line tension values. The tensile strength of CF increases quickly below 1200 degrees C and then slowly above 1200 degrees C as the carbonization temperature raises. Even though during carbonization, a proper relaxation should also be explored in order to prepare optimal CF. (C) 2008 Wiley Periodicals, Inc.