The ability to form external chromia scales on binary Cu-Cr alloys with very small mutual solubility of the two components is strongly increased either by increasing Cr content or by preparing alloys with a very small grain size. The purpose of the present work is to mainly examine the effect of Cr content and especially the influence of the size of the second phase. Equal channel angular pressing (ECAP) was carried out for the grain refinement because it can often provide significant inner deformation and very fine grains. The oxidation behavior of binary Cu-Cr alloys with different nominal Cr contents (Cu-0.5Cr, Cu-7.0Cr and Cu-15.0Cr, atomic fraction, %) was investigated in air at 700 and 800 ℃. At the same time, the oxidation of grain-refined Cu-7.0Cr alloy was compared with the same casting alloy with a normal grain size in order to further reveal the effect of the grain refinement on the oxidation. The oxidation kinetics of all alloys followed the parabolic law. Oxidation of Cu-0.5Cr alloy was basically similar to that of pure Cu and its scales are mainly composed of copper oxides containing a small amount of chromia particles dispersed in the inner layer, even close to the scale/alloy interface. The oxide scales formed on the Cu-7.0Cr and Cu-15.0Cr alloys were complex and were consisted in most cases of the outer layer of CuO and Cu₂O plus inner layer of mixed oxides of chromia and double Cu-Cr oxide of Cu₂O·Cr₂O₃, leaving unoxidized Cr particles surrounded by chromia in the scales. Cr depletion was also observed in the alloy. The grain-refined Cu-Cr alloy easily formed more chromia with much lower oxidation rate. The oxidation rate of Cu-Cr alloys decreased considerably with increasing Cr content and reduction in size of β phase is favorable for improvement of anti-oxidation of Cu-Cr alloys. The result indicates that the alloy microstructure affects the oxidation behavior because microcrystalline structures provide numerous diffusion path for reactive Cr component, shorter diffusion distance and rapid dissolution of Cr-riched second phase. All of these favor the formation of the stable chromia. Therefore, it can be deduced that the growth law and microstructure of the oxide scales for the binary alloy are closely related to the reactive component contents, original microstructure, the size and spatial distribution of β phase in Cu-Cr alloys.