Structural evolution and crystallization behavior between 600 degrees C and 1450 degrees C during the preparation of bulk SiC/B4C/C nanocomposites by the pyrolysis of CB-PSA preceramic were investigated. The CB-PSA preceramic converts into carbon-rich Si-B-C ceramics up to 800 degrees C. Structural evolution and crystallization of Si-B-C materials could be controlled by adjusting the pyrolytic temperature. The Si-B-C ceramics are amorphous between 800 degrees C and 1000 degrees C. Phase separation and crystallization begin at 1100 degrees C. The crystallization of beta-SiC takes place at 1100 degrees C and B4C nanocrystallites generate at 1300 degrees C. The sizes of beta-SiC and B4C nanocrystals increase with the pyrolytic temperature rising. In addition, the boron-doping effect on structural evolution was studied by comparing the crystallization and graphitization behavior of Si-B-C ceramics and the corresponding Si-C materials. Boron is helpful for the growth of beta-SiC nanocrystals and the graphitization, but harmful for the nucleation of beta-SiC crystallites.