Abstract: The corrosion behavior of TP100 steel in CO2 and O2 coexisting condition was studied using weight loss method, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). The results show that the CO2-O2 corrosion rate of TP100 steel is remarkably higher than that of CO2 corrosion or O2 corrosion, respectively. CO2-O2 corrosion scale is loose, composed of FeCO3, Fe2O3 and Fe3O4, and lots of pores exist in the corrosion scale. The porous scale provided fast transferring channel for corrosive solution, and as a result, obvious pits appeared on the surface.