The oxidation of binary Co-5Al alloys in 1 atm of pure O2 at 700 and 800°C was investigated. Oxide scales generally showed good adherence to alloys during oxidation process, although scale spallation on cooling was observed inevitably due to difference in the thermal expansion coefficients between oxides and alloys. It indicated that oxidation of the two alloys approximately obeyed parabolic rate law (n=2). The parabolic rate constants as functions of temperature at 700 and 800°C were 8.38×10-11, 3.45×10-10 g2cm-4s-1, respectively. The binary Co-5Al alloys presented a non-protective oxidation behavior because the Al content was not enough to support the growth of a dense, stable Al2O3 layer. The scales were mainly composed of oxides of cobalt, coupled with a little Al2O3 present at the inner part of the scales.