When MgO+ZrO2 were used as a sintering aid to SiC ceramics, it was found that ZrO2 reacted with SiC and MgO to form ZrC and a magnesium silicate. This reaction may provide a new route to synthesize ZrC or prepare ZrC/SiC composites by reaction sintering using ZrO2, SiC, and MgO as starting materials. In the present work, formation process of the ZrC phase in the SiC-MgO-ZrO2 system was addressed. Subsolidus phase relations at 1550 degrees C of the SiC-MgO-ZrO2 system and the related SiC-SiO2-ZrC-ZrO2-MgO (Si-Zr-Mg-O-C) system were investigated.