The investigation of oxidation kinetics and oxide scale formation on heat-treated Fe-Ni-Cr alloys at high temperature isothermal oxidation was studied. Altering the grain size of the Fe-Ni-Cr alloy through heat treatment was performed to investigate its impact on oxidation characteristics. Heat treatment was carried out at 1000 ºC and 1200 ºC. Subsequently, the heat-treated alloy was subjected to a 150-h isothermal oxidation test at 700 ℃. The investigation focused on characterizing the grain structure of heat-treated samples using the linear intercept method, and then utilized oxidation kinetics and X-ray diffraction techniques to analyze the behavior of heat-treated alloys post-oxidation. As the temperature of the heat treatment increases, the grain structure also grows. The oxidation rate of heat-treated Fe-Ni-Cr alloys shows both parabolic and linear rate laws. Heat treatment at 1000ºC is preferred as these samples exhibit a diffusion-controlled mechanism and a parabolic rate law, preventing further oxidation. The sample treated at 1200ºC displays a linear rate law, resulting in continuous formation and growth of the oxide layer. Furthermore, the oxidation test led to the formation of several oxide phases, including Cr-rich, Cr-Mn-rich and Ti-rich oxide layers.