Mechanism of the oxidation of the thermal-resistant Fe-C-Si-Al-Zr alloys was investigated at high temperatures. For the analysis three series samples: L1, L2 and L3, had been annealed at 1100 deg C, were cast. After annealing the samples were investigated on the oxide thickness increase. They were observed and analysed by optical and electron microscopes. In case of the small zirconium content in the L1 sample, the graphite lamellae and only a significant amount of zirconium carbide in the ferrite matrix have been observed. At higher zirconium content (the L2 and L3 samples) graphite lamellae and particles of ZrC have been observed. During annealing an oxidation of graphite and ZrC took place. The oxidation of ZrC into ZrO sub 2 is direct and progressive. Evolving CO gas formed the pores around ZrO sub 2 . Under sufficient amount of the dissolved oxygen the oxidation of aluminium was taking place too. Al sub 2 O sub 3 was precipitating in the CO-ferrite interface filling up the formed pores. Finally the interaction between ZrO sub 2 and Al sub 2 O sub 3 were taking place and formation the compound of ZrO sub 2 Al sub 2 O sub 3 retarded further oxidation.