Isothermal oxidation behavior of CoNiCrAlY(Re) alloy before and after heat-treatment at 1000 oC in air was investigated by X-ray diffraction, scanning microscopy and energy spectroscopy analysis. The results show that heat-treatment favors the homogeneous distribution of Al-rich phase in the alloy. Heat-treatment induces alloys to enter steady oxidation stage faster and lowers the oxidation rate. Compared with Re-undoped alloy, Re-doped alloy has higher oxidation mass gain during the initial oxidation stage and earlier phase transformation of alumina from metastable θ-Al2O3 to stable α-Al2O3.