The work aims to further enhance the high temperature oxidation resistance of new low density refractory high-entropy alloy (RHEA) NbMoCrTiAl-1Si. NbMoCrTiAl-1Si-xB (x=0, 1) was prepared by non-consumable vacuum arc melting technology and the high temperature oxidation tests were conducted at 1173 K for 48 h in METTLER TOLEDO TGA/DSC1 thermogravimetric simultaneous thermal analyzer. The microstructure and high temperature oxidation resistance of the alloy were investigated by XRD, SEM and EDS. The results showed that the microstructure of as-cast NbMoCrTiAl-1Si-xB (x=0, 1) alloy had the characteristic of dendritic morphology. The addition of 1% B did not change the dendritic microstructure, but transformed the interdendritic microstructure from the black nanoparticles in NbMoCrTiAl-1Si alloy into the bright short- rod shape in NbMoCrTiAl-1Si-1B alloy. The oxidation process of NbMoCrTiAl-1Si alloy was divided into the initial linear rapid oxidation stage and the following parabolic slow oxidation stage with the total oxidation weight gain of 1.064 mg/cm2, whereas the oxidation process of NbMoCrTiAl-1Si-1B alloy consisted of parabolic slow oxidation stages with different oxidation rate constants and its total oxidation weight gain was about one-half of that of NbMoCrTiAl-1Si alloy. After oxidized at high temperature for 48 h, the obvious internal corrosion was observed in the interdendritic zone of NbMoCrTiAl-1Si alloy and the rough oxide film was formed on the surface mainly with the flaky and needle-like Al2O3 in the dendritic zone and TiO2 clusters in the interdendritic zone, while the continuously dense flat oxide film was formed on the surface of NbMoCrTiAl- 1Si-1B alloy mainly including Al2O3, TiO2 and Ti0.4Al0.3Nb0.3O2. Therefore, the addition of B can accelerate the formation of the dense oxides of Al2O3 and TiO2 on the surface of NbMoCrTiAl-1Si-1B alloy, thus inhibiting the internal oxidation, and greatly enhancing the oxidation resistance of NbMoCrTiAl-1Si-1B alloy.