A Ni-based alloy Ni-(12~18) Cr, (10~14) Co, (2~6) W, (2~6) Mo, (2~6) Ti, (2~6) Al was prepared via hot isostatic pressing by 150 MPa at 1150 ^oC with its argon gas atomization powders as raw material. Then its oxidation behavior in air at 950 ^oC was studied by intermittent weighing method in terms of the oxidation kinetics, phase composition, microstructure and elemental distribution of oxide scale and characteristics of oxygen-affected zone etc. The results show that the oxidation resistance of the as-HIPed superalloy at 950 ^oC has reached the full oxidation resistance grade in accord with the national standards. Both the oxidation kinetics curve and the depth variation curve of oxygen-affected zone follow a parabolic law. In the early stage of oxidation, Ti and Al atoms in the matrix diffuse rapidly along the prior particle boundaries (PPB) towards the alloy surface and form a continuous mixed oxide layer of Cr₂O₃, TiO₂ and Al₂O₃. However, oxygen atoms diffuse inward along the PPB and undergoes in-situ oxidation reaction with secondary phases to form granular internal oxide Al₂O₃. After 24 hours oxidation, the surface oxide scale consists of the outmost layer of mixed oxides TiO₂ and Al₂O₃, and the inner layer of Cr₂O₃ oxide. The continuous internal oxides TiO₂ and Al₂O₃ appear in Cr-depleted zone. In the later stage of oxidation (> 100 h), the outmost TiO₂ oxide layer peels off from the alloy surface, and the internal oxides TiO₂ and Al₂O₃ distributed in Cr-depleted zone in a continuous root-like manner. Finally, the high temperature oxidation mechanism and formation process of internal oxides in as-HIPed nickel-based superalloy was also discussed.