This work aims to develop a low cost Fe-Cr-Ni medium entropy alloy with excellent high temperature oxidation resistance, and then, the microstructure and micro-mechanical distribution of oxide layer formed on Fe-Cr-Ni medium entropy alloy were studied. The oxidation kinetics in the air at 1 150-1 240 ℃ was obtained by continuous oxidation and weight gain experiment. The morphology, composition and phase composition of the oxide film oxidized at 1 150 ℃ for 4 h were analyzed by SEM, EDS, XRD and XPS techniques. The micro-mechanical distribution of oxide layer was studied by micro-scratch method. The result show that, the oxidation resistance of Fe-Cr-Ni medium entropy alloy is excellent with a high oxidation activation energy that is 417.64 kJ/mol. The weight gain at high temperature in air follows a parabolic rule. After being oxidized at 1 150 ℃ for 4 h, the entropy alloy oxide film of Fe-Cr-Ni formed a double-layer structure. The outer oxide layer was mainly composed of dense Mn(Fe/Cr)2O4 spinel, Fe2O3 and a small mount MnO2 oxides, the inner oxide layer was composed of dense and continuous Cr2O3. Meanwhile, internal oxide particles SiO2 were dispersed at the interface between the matrix and the oxide film. The micro-mechanical distribution characteristics of the oxide layer reflect its layered structure, the binding force between the outer oxide layer and the inner oxide layer is about 10 N, while the binding force between the inner oxide layer and the matrix is about 24 N. With the increase of scratch depth, the indentation moves from outer oxide layer to matrix, the plastic deformation ratio decreases, while the fracture toughness increases from outer oxide layer to matrix. The developed low cost Fe-Cr-Ni medium entropy alloy really show an excellent high temperature oxidation resistance. After high temperature oxidation, the dense oxide layer formed on the surface effectively hinders the inward diffusion of O2? and the outward diffusion of alloying elements. Meanwhile, the internal oxide particles in SiO2 increase the binding force between the oxide layer and matrix, this is an important reason for the excellent oxidation resistance of the entropy alloy in Fe-Cr-Ni. The dense and continuous Cr2O3 layer effectively impedes the growth of the oxide layer, while the internal oxide particles SiO2 increase the binding force between the oxide film and the matrix, which is an important reason for the excellent oxidation resistance of Fe-Cr-Ni MEA.