Amorphous boron-rich SiBCN were prepared by high-energy ball-milling of the mixtures of Si, graphite, h-BN, and inorganic boron, which acted as extra boron source. The solid-state amorphization, thermal stability, and crystallization of the boron-rich SiBCN were studied in detail. It was suggested that mechanical alloying can drive solid-state amorphization but also can be an initiation step for the nucleation of nanocrystals. The amorphous networks of Si-C, C-B, C-C, C-N, B-N, and C-B-N bonds are detected by XPS; however, solid-state NMR further confirms the formation of a new chemical environment around B atoms, BC3. The increases in boron content improve the thermal stability of SiBCN ceramics but weaken their oxidation resistance. Nano-SiC crystallizes first while BN(C) forms subsequently. Boron promoting SiC crystallization may result from the reduced hindering effects of B-N-C nanodomains that retard SiC crystallization.