Structural evolution and crystallization behavior between 600°C and 1450°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°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°C and 1000°C. Phase separation and crystallization begin at 1100°C. The crystallization of β-SiC takes place at 1100°C and B4C nanocrystallites generate at 1300°C. The sizes of β-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 β-SiC nanocrystals and the graphitization, but harmful for the nucleation of β-SiC crystallites.