The oxidation kinetics, surface morphology and phase structure of oxide films grown on 25Cr20Ni alloy in air-H₂O and H₂-H₂O atmospheres at 900 ℃ for 20 h were investigated. The anti-coking performance and resistance to carburization of the two oxide films were compared using 25Cr20Ni alloy tubes with an inner diameter of 10 mm and a length of 850 mm in a bench scale naphtha steam pyrolysis unit. The oxidation kinetics followed a parabolic law in an air-H₂O atmosphere and a logarithm law in a H2-H₂O atmosphere in the steady-state stage. The oxide film grown in the air-H₂O atmosphere had cracks where the elements Fe and Ni were enriched and the un-cracked area was covered with octahedral-shaped MnCr2O4 spinels and Cr1.3Fe0.7O3 oxide clusters, while the oxide film grown in the H2-H₂O atmosphere was intact and completely covered with dense standing blade MnCr2O4 spinels. In the pyrolysis tests, the anti-coking performance and resistance to carburization of the oxide film grown in the H2-H₂O atmosphere were far better than that in the air-H₂O atmosphere. The mass of coke formed in the oxide film grown in the H2-H₂O atmosphere was less than 10% of that in the air-H₂O atmosphere. The Cr1.3Fe0.7O3 oxide clusters converted into Cr23C6 carbides and the cracks were filled with carbon in the oxide film grown in the air-H₂O atmosphere after repeated coking and decoking tests, while the dense standing blade MnCr₂O4 spinels remained unchanged in the oxide film grown in the H2-H₂O atmosphere. The ethylene, propylene and butadiene yields in the pyrolysis tests were almost the same for the two oxide films.