In order to obtain the diagrams of stability relations (namely, predominance area phase diagram, PAPD) for carbothermal reduction and nitridation (CRN) of zircon, the thermodynamic calculation and the analysis on the Zr-Si-C-N-O system were carried on by the method of , and PAPD of the Zr-Si-C-N-O system was constructed. The CRN of zircon may lead to the formation of different oxide-nitride composites such as ZrO2-Si2N2O, ZrN-Si2N2O, and ZrN-Si3N4 by controlling reaction conditions of carbon proportion, temperature, and partial pressures of CO and N2. Zircon powder (45 μm), activated carbon and N2 (99.999%) were used as the starting materials, the effect of carbon proportion on the mass loss and the product phase composition after CRN of zircon was investigated. Carbon proportion affects the yielded reaction products obviously; and the higher the carbon proportion, the lower the starting temperature for the CRN to occur.