Abstract: The relationship of creep rupture life and microstructural evolution under different high temperature oxidation conditions for Cr35Ni45 Nb alloy was studied by using SEM and quantitative electronic probe analysis. It was found that the microstructure gradually varied both in the edge and inside of the tube with increasing treatment time. The oxidation sequence showed that discontinuous oxidation films firstly developed at different places from the surface of the tube specimen,then they became continuous,and the carbide depleted zone developed as well during the high temperature oxidation process. As time went by,the oxidation films ruptured with temperature fluctuation,the characteristic of a higher oxygen partial pressure during the aging process resulted in the untimely recovery of the broken oxide films,and internal oxidation occurred at grain boundaries. Furthermore,creep rupture tests under different pretreating conditions of oxidation in air and argon gas atmosphere showed that the newly-built thin and continuous oxidation films improved the creep properties,while with increasing aging time,carbide depleted zone formation,oxidation film rupture and internal oxidation could decrease the creep properties of this alloy.