A conventional cast nickel-base superalloy with the chemical composition of Ni-16W-2Mo-1.5Cr-6Al wt% has been recently developed as isothermal forging die material operating at temperatures above 1000 degrees C. The oxidation test results showed that the oxide scale spalled heavily after exposure in air at 1050 degrees C for 100h, which is unacceptable for the process of isothermal forging. To improve the high temperature oxidation resistance, 3 wt% Ta was added into the alloy. Specimens after oxidation test were analyzed by the scanning electron microscope (SEM) and the X-ray diffraction analysis (XRD). The results showed that the oxides were mainly NiO, NiWO4, and NiAl2O4 for all the samples. The oxidation resistance of the alloy with 3 wt% Ta was fairly good at 1050 degrees C with the mass gain rate of around 0.016g/m(2)center dot h and spallation of oxides less than 2g/m(2) within 100h. The SEM results showed that a continuous protective layer of Al2O3 has been formed under the layer of NiO for the alloy containing Ta.