This project studies the influence of different grain sizes of Ni-based Fe-33Ni-19Cr alloy obtained from heat treatment procedure on high temperature isothermal oxidation. Heat treatment procedure was carried out at two different temperatures, namely 1000℃ and 1200℃ for 3 hours of soaking time, followed by quenching in the water. These samples are denoted as T1000 and T1200. The heat-treated Ni-based Fe-33Ni-19Cr alloy was subjected to an isothermal oxidation test at 950℃ for 150 hours exposure. Oxidized heat-treated alloys were tested in terms of oxidation kinetics, phase analysis and surface morphology of oxidized samples. Oxidation kinetics were determine based on weight change per surface area as a function of exposure time. Phase analysis was determined using the x-ray diffraction (XRD) technique and surface morphology of oxidized samples was characterized using a scanning electron microscope (SEM). As a result, the heat treatment procedure shows varying grain sizes. The higher the heat treatment temperature, shows an increase in grain size with a decrease in hardness value. The oxidation kinetics for both heat-treated samples showed an increment pattern of weight change and followed a parabolic rate law. The oxidized T1000 sample recorded the lowest parabolic rate constant of 3.12×10^–8 mg²cm^–4s^–1, indicating a low oxidation rate, thus having good oxidation resistance. Phase analysis from the XRD technique recorded several oxide phases consisting of Cr₂O₃, MnCr₂O₄, and (Ti_0.97Cr_0.03)O₂ oxide phases. In addition, a uniform oxide layer is formed on the oxidized T1000 sample, indicating good oxide scale adhesion, thereby improving the protective oxide behavior.