Three U720Li ingots with different B contents were produced by vacuum induction melting and afterwards they were fully homogenized. The effect of B on the isothermal oxidation behavior of as-homogenized U720Li alloy at 1100 ℃ was investigated by a discontinuous increasing weight method. After the homogenization treatment, extremely serious B depletion from the alloy matrix was found by ICP-AES, indicating that B could diffuse rapidly towards the oxide scale during high temperature air exposure (above 1100 ℃). The B addition decreased the oxidation rate and reduced the volume fraction of pores and the thickness of γ′ depleted layer. The scales formed on both low B and high B alloys consist of three layers, loose outer layer composed of (Ni/Co)O, TiO2 and complex spinel, compact intermediate layer is mainly Cr2O3 and discrete internal oxides are Al2O3+TiO2. It is deemed that B improves the oxidation resistance mainly by promoting the formation of a more continuous and protective Cr2O3 intermediate layer.