To investigate the oxidation behavior of a nickel-based superalloy with high hafnium content(1.34 wt%), this study performed isothermal oxidation tests at 900, 1000, and 1100℃ for up to 200 h. X-ray diffraction and scanning electron microscopy with energy-dispersive X-ray spectroscopy were applied to study the oxidation behavior. The weight gain of the high Hf nickel-based superalloy exhibited a parabolalike curve, and no spallation of the oxide scale was observed during the oxidation tests. The alloy presented excellent oxidation resistance, and no HfO_2 was observed in the oxide scale at 900℃. With the increase of the oxidation temperature to 1000℃, HfO_2 particles formed in the spinel phases of the scale, and "peg-like" HfO_2 was observed within and beneath the inner layer of Al_2O_3 after 200h. As the oxidation temperature rose to 1100℃, "peg-like" HfO_2 was observed at the early stage of the oxidation test(within 25 h). The formation mechanism of HfO_2 and its impact on oxidation resistance were investigated based on the analysis of the oxidation test results at different temperatures.