The isothermal oxidation behavior of a new Ni-Cr-W base superalloy Ni-20Cr-18W at 1100 oC has been investigated by analytical electron microscopy, scanning electron microscope coupled with energy dispersive spectrum and X-ray diffraction. Oxidation kinetics was evaluated from measurements of mass change, metal loss and internal penetration. Initially, the mixtural oxide scale is composed of hexagonal Cr2O3, cubic NiO and spinel-type (Ni，Mn，Cr)3O4 isomorphous with M3O4 at the initial stage of oxidation. After long-term oxidation, the oxide scale turns from mono-layer to double-layer. There is a consecutive Cr2O3 scale at inner adherent layer, and a compact NiO scale at outer layer which can restrain the inner Cr2O3 volatilization. At the same time, the oxidation void has formed between the oxide scale and interface of the alloy (scale/metal interface), and the inner oxidation of Al element happens at this place