The isothermal oxidation behavior of magnetron-sputtered Cr-coated Zr-1Nb alloy cladding in 1000–1300 ℃ steam environment is studied. The phenomena at each temperature are summarized and the oxidation kinetics are discussed. Cr coating can maintain the oxidation resistance for more than 6 h at 1000 ℃ and 1100 ℃ under double-sided oxidation condition, while suffered degradation during long-duration oxidation at 1200 ℃ and 1300 ℃. After coating degradation, Zr substrate was oxidized prior to residual metallic Cr and the metallic Cr was not completely oxidized until the ZrO2 layer beneath the coating had grown to a certain thickness. Mainly due to the redox reaction between Zr and Cr2O3 and the preferential oxidation of Zr substrate, the thickness of Cr2O3 film decreased after a certain oxidation duration. The Cr coating after degradation at 1200 ℃ can still hinder the oxidation of Zr substrate to a certain extent, while that at 1300 ℃ almost lost the protective effect. •Isothermal steam oxidation of Cr-coated Zr alloy was performed in temperature range of 1000–1300 ℃.•Oxidation kinetics of Cr-coated Zr alloy cladding was summarized.•Oxidation behavior of Zr substrate after coating degradation were discussed.