The crystallization behavior of amorphous Si2BC3N monoliths by heating at 1000 degrees C-1400 degrees C and 5 GPa was investigated with the special attention to the nucleation mechanisms of beta-SiC and BN(C) phases. Nanoscale puckered structures arising in particle bridging areas were found and its evolution behavior well reflected the nucleation process of nanocrystallites. The temperature-dependent crystallization of amorphous Si2BC3N monoliths at 5 GPa passes through four stages: The material remains amorphous below 1100 degrees C. It undergoes partial phase segregation (1100 degrees C-1200 degrees C), followed by initiation of nucleation (1200 degrees C-1250 degrees C), and then nucleation and growth of beta-SiC and turbostratic BN(C) crystallites (>1250 degrees C). The first principles calculation indicates the nucleation precedence of BN(C) phase over beta-SiC. BN(C) nucleates preferentially at bridges between ceramic particles causing SiC to concentrate in particle interiors thus forming capsule-like structures.