The present work investigated the phase relations in SiC-Al₂O₃-Y₂O₃-SiO₂ (Si-Al-Y-O-C) system. As a continuation of our previous works, the purpose of this study is to understand the high temperature reaction behaviors of SiO₂ in the system and its effect on the phase relations of the valuable system of SiC-Al₂O₃-Y₂O₃. The phase compositions of six solid-state reacted samples with different components of Y₂O₃:Al₂O₃:SiC:SiO₂ were analyzed by XRD. The phase relations of the systems were determined. The subsolidus phase diagrams of ternary Al₂O₃-SiC-SiO₂ system and the tentative phase diagram of an extended quaternary Y₂O₃-Al₂O₃-SiC-SiO₂ system were presented latter involving several coexisting regions of four phases. The high temperature reaction behavior of SiO₂ in the system and its effect on the phase relations of system were discussed.