Al₂O₃-based directionally solidified eutectic (DSE) ceramics are recognized as promising candidates for high-temperature structural materials in advanced aeroengines. Nevertheless, their corrosion resistance at elevated temperatures continues to pose a critical challenge, limiting broader application in hot-section components. This study investigates corrosion behavior of RE₃Al₅O₁₂ (REAG)/Al₂O₃ (RE = rare earth) DSE ceramics in water vapor atmosphere (90 vol.% H₂O(g) + 10 vol.% air(g)) at 1500℃ for durations up to 200 hours, with focus on the influence of eutectic structure and RE elements in garnet phases via examining three samples (high-entropy (Y_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₃Al₅O₁₂ DSEs fabricated at 10 and 300 mm/h and YAG/Al₂O₃ DSE grown at 10 mm/h). The results indicate that REAG/Al₂O₃ DSE ceramics exhibit excellent water vapor corrosion resistance at 1500°C for up to 200 hours, with mass loss values ranging from -0.00757 to -0.00708 mg·cm^-2·mg^-1. During corrosion, Al₂O₃ phase acts as corrosion-susceptible component compared to REAG phase, with corrosion depth showing a nearly linear relationship with the average Al₂O₃ lamellar width. In addition, garnet phases experience slight grain growth, reducing the contact area between water vapor and Al₂O₃ phase; Gd demonstrates marginally lower stability compared to other RE elements. Despite these changes, all samples maintain their preferred crystallographic orientations, confirming the structural stability of REAG/Al₂O₃ DSEs under water vapor atmosphere at 1500℃.