In this study, the oxidation and sulfidation characteristics of modified 9Cr-1Mo steels according to Cr-rich M23C6 distributed in grain boundaries in the N-2-O-2-CO2-SO2 atmosphere at 650 degrees C were analyzed. The oxidation kinetic was obeyed by the parabolic law. The oxide layer presented a three-layer structure consisting of hematite, magnetite, and Fe-Cr spinel from the outside. In the initial oxidation stage, Cr-rich M23C6 exposed to the surface acted as nuclei of Cr-rich oxide. However, it is thought to hardly effect on the growth of the outer oxide layer. The Cr-rich M23C6 distributed in grain boundaries preferentially formed a Cr2O3 layer at the oxide/matrix interface, and internal oxidation characteristics were shown to be different depending on the presence or absence of SO2 in the atmosphere.