Materials used in high temperature regions in aviation must be corrosion resistant and have high strength values. Heat treated Ni-based superalloys (Hastelloy X, Inconel 718 and Inconel 738) supplied from the industry were first prepared metallographically, and then their microstructures were visualized under an optical microscope. Then, oxidation process in air was applied to the samples, which were brought to the appropriate size for the oxidation test, with a thermogravimetric analysis (TGA) device at 600 and 900 °C temperatures for 48 hours. The oxide layers formed on the surfaces of the oxidized samples were visualized with a scanning electron microscope (SEM/EDS) and elemental analysis was performed. Diffusion coefficients were calculated using the Arrhenius equation and oxygen diffusion kinetics were determined. The importance of this study is to reveal and compare the oxidation kinetics of some superalloys and to facilitate access to evaluation data in terms of usage that the industry needs.