Three kinds of Al2O3-Si-C matrix specimens were prepared using tabular corundum powder and Si powder as starting materials, ultrafine flake graphite, nano carbon black, and carbon nanotubes as carbon sources, respectively, to research the effect of micro or nano carbon materials on structure and morphology of formed SiC crystals. The specimens were fired at 1 000, 1 200 and 1 400 ℃ for 3 h in carbon-embedded condition, respectively. The phase composition was studied by XRD and the crystal morphology of SiC was investigated by FESEM. The results show that: (1) the amount of SiC increases with the firing temperature rising; (2) the in-situ reaction mechanism and the formed SiC crystal morphology vary with carbon source: carbon nanotubes are generally converted into SiC whiskers by carbon nanotubes-confined reaction; Si and nano carbon black react to nucleate quickly, and the nucleated SiC crystals grow evenly in all directions forming SiC particles; SiC whiskers are produced from edge to internal of ultrafine flake graphite.