Ni-5Al-xSi (x=0, 1.5, 3.0, 4.5, mass fraction/%) superalloys were prepared by powder metallurgy technique, and then isothermally oxidized at 1 000, 1 100℃ for 100 h. The morphology of oxide film on the alloy surface was observed and the phase composition was analyzed. The high temperature oxidation resistance of the alloys was studied. The results show that the high temperature oxidation resistance of the tested alloys increased with increasing silicon addition amount; when the mass fraction of silicon reached 3.0%, increasing silicon content had little effect on high temperature oxidation resistance improvement of the alloys. The oxide film on surface of the alloy without addition of silicon was mainly composed of NiO. After addition of silicon, the oxide film mainly consisted of NiO, NiAl2O4 and Al2O3, and showed spalling features at different degrees. When the mass fraction of silicon was 4.5%, the high temperature oxidation resistance of the tested alloy was the best.