A detailed knowledge of structural evolution during mechanical alloying (MA) is of interest and critical for understanding of synthesis mechanism, optimization of milling process, and control of milling products. The structural evolution of the 2Si-BN-3C mixture with a molar ratio of Si:BN:C = 2:1:3 during MA was studied by investigating the changes of phases, morphologies, elemental distributions, microstructures, and chemical bond states using XRD, SEM-EDS, TEM, XPS, and Raman. With the increases in milling time, the particle sizes of the milled 2Si-BN-3C powders first decrease to submicrometers (similar to 1 hour), slightly increase afterward (similar to 3 hours), and eventually decrease to nanoscales gradually reaching equilibrium (>= 10 hours). Depending on the intrinsic crystal structures of themselves, h-BN and graphite are almost amorphized after milling of 10 hours, while amorphization of c-Si takes at least 20 hours. The same milling parameters can provide amorphous Si2BC3N powders, but incompletely amorphous (partially crystalline) SiC powders, which is mainly due to the dilution effect of B and N atoms on the atomic concentration of Si and C hindering the microdiffusion and subsequent mechanochemical reaction of Si and C. Crystallite refinement-induced amorphization is the major synthesis mechanism of amorphous Si2BC3N powders. This work would offer more insight into the MA synthesis of multiple component materials, especially Si-based brittle systems.