Isothermal oxidation experiments in the range of 450–525 °C were carried out to investigate the effect of different Er contents (0, 0.35, 0.96 and 2.27 wt.%) on the oxidation resistance of Mg-4Al-0.2Mn alloys. The addition of Er extended the incubation period of the alloys. The best oxidation resistance of the alloy was obtained with the Er content of 0.96 wt.%. This is due to (i) the formation of a stable oxide film containing Er2O3 on the surface of the alloy, which inhibits further oxidation of the substrate, and (ii) the improvement of the thermal stability of the substrate. The oxidation kinetics of these alloys at elevated temperatures follow parabolic, linear, and superlinear laws. •The effect of Er addition on the high temperature oxidation resistance of Mg-4Al-0.2Mn alloy was investigated.•The addition of Er reduces the oxidation weight gain and oxidation rate of Mg-4Al-0.2Mn alloy.•The formation of Er2O3 contributes to the formation of a dense oxide film.