The In2O3-SnO2-ZnO system is important for the transparent conducting oxides. The isothermal phase diagrams of the ternary In2O3-SnO2-ZnO system at 1400 and 1500 °C were experimentally studied by quenching experiments followed by XRD and EPMA phase analysis. A new ternary In2Sn2Zn2O9 phase with crystal structure analogous to R3c ZnSnO3 was found, and its eutectoid decomposition reaction to In2O3 bixbyite s.s. + Zn2SnO4 spinel s.s. + SnO2 was determined to be 1310 ± 10 °C. In addition, a small homogeneity range of this ternary phase toward In2O3 was found: In2+2xSn2-xZn2-xO9 where x = 0.0 ∼ 0.2 by Zn2+ + Sn4+ = 2 In3+ substitution.