Abstract: Cr12 ferritic stainless steels with different amounts of cerium were obtained, their high temperature oxidation resistance was tested according to the standard HB 5258-2000, oxidation phenomena and products phase analysis were studied by SEM and XRD, and thermodynamic calculations of the formation of oxidation products were carried out. Experimental results show that the oxidation analysis results consist with the thermodynamic calculation ones. The primary oxidation product is M₃O₄ type spinel oxide at lower temperatures of 600℃, 700℃ and 800℃, which has good protective capability, and this protective film can be accelerated by adding cerium. As the result, the oxidation rate can be decreased at these temperatures. At 900℃, the primary oxidation product is M₂O₃ type oxide, which has poor protective capability, and cerium-containing steels with refined grain size are oxidized faster at the beginning. Whereas cerium can improve the adherence between oxide and substrate, prevents materials from being further quick oxidized in the later period. The oxidation mechanism was well explained by reactive element effect and grain size effect.