The phase relations of the ternary systems SiC-Si3N4-R2O3 (R = Gd and Y) have been investigated by XRD phase analyses of the hot-pressed samples from SiC, Si3N4, and R2O3 powders. Their subsolidus phase diagrams were presented. In situ SiO2 impurity in the powders always led to form some oxygen-richer rare-earth silicon-oxynitrides and extend the ternary systems to the quaternary systems of SiC-Si3N4-SiO2-R2O3 (R = Gd, Y). Within these systems all rare-earth siliconoxynitrides coexist with SiC. The phase diagrams of the quaternary systems of SiC-Si3N4-SiO2-R2O3 (R = Gd, Y) were established, in which the subsolidus diagram of Si3N4-SiO2-Gd2O3 system was first reported.