The phase and microstructure evolutions of multiwalled carbon nanotubes (MWCNTs) in B4C- and Si-containing Al2O3–C specimens under elevated temperatures were investigated by means of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results show that the incorporation of B4C decreases the partial pressure of SiO(g) in Al2O3–C specimens due to the oxidation of B4C prior to Si at lower temperature, which prevents the transformation of MWCNTs at 1000°C and suspends the transformation under higher temperature. B2O3 vapor resulting from oxidation of B4C powder reacts with C sources to generate nanoscaled B4C droplets, which facilitate the catalytic formation of new MWCNTs and nano onion-like carbon. In addition, B-doped MWCNTs and BN tubes with the coexistence of B2O3, MWCNTs, and N2 are obtained under evaluated temperatures.