The isothermal oxidation behaviors of two kinds of single-crystal nickel-base superalloys DD10 with different cobalt contents were investigated at 900, 1000 and 1100 oC in air by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy disperse X-ray analysis (EDX). It is found that increasing the cobalt content can decrease the diffusion activity energy of the alloy and thus increase the oxidation rate slightly. For the oxidation at 900 oC and 1000 oC, the mass gain basically obeys the parabola law. The oxide scales consist of three layers: an outer layer of Cr2O3 and TiO2, a thin intermediate layer of CrTaO4 and Ta2O5, and a continuous inner layer of Al2O3. For the oxidation at 1100 oC, because of the severe spalling of the scale and the volatilization of the CrO3, the kinetics curves deviate from the parabolic law a little. Some spinels such as NiCr2O4, CoAl2O4, CoNiO2 and Co2TiO4 form and inner nitridation phenomenon happens