Abstract: The isothermal reaction rate of domestic Zircaloy--4 in flowing steam is measured in the temperaurerange of 900--1500℃. The reaction rate measured from the weight gain of the sample follows theparabolic law above 1000℃, the growth rates of ZrO_2 layer, oxygen--stablized alpha phase α--Zr (O)layer and ZrO_2 + α--Zr (O)layer are also parabolic above 1000℃. The parabolic rate constants are K_p(O_2) = 4.98×10~5exp(--20907/ T ), mg~2·cm~(-4)·s~(-1); K_p (ZrO_2) = 0. 0104 exp(--17592/ T ),cm~2·s~(-1); K_p(α--Zr (O) ) = 0.6407exp(--23207/ T ),cm~2·s~(-1); K_p(ξ) = 0. 3025exp(--20194/ T), cm~2·s~(-1). The oxide film produced in high temperature water decreases Zircaloy--steam reaction ratesat 1100℃ or less. Different surface treatments(pickling and polishing)and different heat treatments (αannealing and β quenching) have small or neglible effects. Oxidation rates of Zircaloy--2 and Zircaloy--4in high temperature steam haven't distinct difference, but the high tempetature steam oxidation resis-tance of Zr--2.5Nb is superior to that of Zircaloy--4 and Zircaloy--2. It is found that when the oxideweight gain of Zircaloy--4 in high tempetature steam in larger than 35mg·cm~(-2), the oxide film easilyseperates and spalls.