The isothermal oxidation kinetics of a Co-40Cr alloy and its yttrium ion-implanted samples were studied at 1 000 ℃ in air by thermal-gravity analysis (TGA). Scanning electronic microscopy (SEM) was used to examine the Cr2O3 oxide film's morphology after oxidation. An acoustic emission (AE) method was used in situ to monitor the cracking and spalling of oxide films formed on samples during oxidation and subsequent aircooling stages. A theoretical model was proposed relating to the film fracture process and was used to analyze the acoustic emission spectrum on time domain and the AE-event number domain. It was found that yttrium implantation remarkably reduced the isothermal oxidation rate of Co-40Cr and improved the anti-cracking and anti-spalling properties of Cr2O3 oxide film. The reasons for the improvement were mainly that the implanted yttrium reduced the grain size of Cr2O3 oxide, increased the high temperature plasticity of oxide film, and remarkably reduced the number and size of Cr2O3/Co-40Cr interfacial defects.