The study examined the oxidation rate law of Fe-Ni-Cr series Ni-based alloy, which underwent an isothermal oxidation test. Fe-Ni-Cr alloy samples with small grain (SG) and coarse grain (CG) structures were produced through heat treatment at varying temperatures. After the heat treatment, the SG and CG samples were subjected to isothermal oxidation at 900 ºC for 300 h. The analysis of the oxidation rate was conducted at 100 h, 200 h, and 300 h exposure durations to observe the behaviour of the oxidized samples. This was achieved by plotting the oxidation kinetics, which involved examining the change in weight per surface area over time. After 100 h of exposure, both SG and CG samples followed a linear rate law, transitioning to a parabolic rate law at 300 h of exposure. SG samples demonstrated superior oxidation behaviour compared to CG samples, exhibiting the shift to a parabolic rate law after only 200 h of exposure. Furthermore, the SG sample displayed higher oxidation resistance as evidenced by its lower parabolic rate constant.