Three SiBCN monoliths: carbon-lean, -moderate, and -rich monoliths with the same Si/B/N mole ratio have been prepared by mechanical alloying followed by reactive hot pressing. The role of carbon on phase composition, microstructural evolution, and mechanical properties were investigated. XRD and TEM investigations showed that the as-sintered monoliths contain several components: Si metal, SiC, t-BN, and BN(C), strongly correlated with the carbon content. TEM results also revealed that BN(C) enwraps SiC grain forming capsule-like structure. Raman, NMR, and HRTEM studies suggested that the structural evolution within the BN(C) areas includes the graphitization of amorphous carbon and the lateral growth of BN(C), accompanied by an increase in the defect concentration. The defects form within BN (0002) basal planes resulting from the disordered carbon imbedded, which in turn determines the morphological evolution of BN(C). The ill-developed microstructures for carbon-lean monoliths and the excessive carbon in carbon-rich ones both deteriorate their mechanical properties and densities.