B4C-SiC nanocomposite powders were synthesized successfully through mechanochemical processing with B4C. Si and graphite powders. Structural evolution of the powder mixtures was characterized by XRD and IR spectra. Microstructure investigations of the end products were performed in HRTEM. Phase transition of the end products associated with heat treatment was monitored by DSC and XRD. The results indicate that the end products are composed of nanocrystalline B4C and amorphous SiC. The synthesis of the amorphous SiC is only controlled by the gradual diffusion reaction mechanism. During ball milling, the C atoms are firstly dissolved into the interstices of Si lattice to form an interstitial solid solution Si (C). After continuous milling, the C atoms located into the interstices of Si lattice gradually occupy the positions of Si to form amorphous SiC. However, no C or Si atom is dissolved into the interstices of B4C lattice. The large quantity of B4C dilutes the reactants of graphite and Si and hinders the self-propagating reaction to form crystal SiC. The end products (nanocrystalline B4C and amorphous SiC) are gradually transformed into crystalline B4C and SiC during heat treatment. (C) 2014 Elsevier B.V. All rights reserved.