In order to discuss the influence of grain refinement on the oxidation behavior of binary two-phase alloys, Cr-25Nb alloys with different grain sizes were prepared by arc-casting and mechanical alloying, and their oxidation behaviors at 950 and 1200 oC in air were investigated. The results show that in both alloys a single and continuous Cr2O3 film did not form, but a complex oxidation films occurred which contained an outer layer of Cr2O3 plus an inner layer of NbCrO4. The oxidation resistance of mechanically alloyed Cr-25Nb was higher than that of the cast alloy. In particular, the mass gain of the cast alloy is twice as much as that of mechanically alloyed alloy when they were oxidized at 1200 oC for 100 h. The differences observed above are attributed to that fine grains can accelerate the relaxation of the oxide scale stress and increase the adhesion of the oxide layer on the matrix