The effects of Si and Nb on the oxidation behavior at 1073 K in high purity 17Cr steels were investigated in conjunction with the Laves phase. In the early stage of oxidation, mass gains increased not only slightly with Nb addition but also further with the addition of Si and Nb, and precipitation of the Laves phase occurred for a short time with Nb addition and was promoted with Si addition. The increase in mass gain in the early stage of oxidation was seemingly caused by the oxidation of the Laves phase near the surface of steel. The parabolic growth rate constant became small for long-term oxidation with Nb addition in spite of Si addition. The reason for this appeared to be that the Laves phase, which had already precipitated at grain boundaries, suppressed the grain boundary diffusion of cations in steel.