SiBN fibers are one of the most admirable microwave-transparent reinforced materials for high Mach number aircrafts. Currently, the detailed high-temperature oxidation behavior of SiBN fibers has not been studied yet. In this work, we studied the high-temperature oxidation behavior of SiBN fibers with different boron contents at the temperature range of 1000-1400 ℃ in air. SiBN fibers started to be oxidized at 1100 ℃, with Si₃N₄ and BN phase oxidized to SiO₂ and B₂O₃, respectively. Due to the gasification and the escape of molten B₂O₃ at high temperatures, amorphous SiO₂ could be remained at the fiber surface. As the fiber further oxidized, the molten B₂O₃ at the inside may infiltrate into the fiber interior to react with Si₃N₄, causing the precipitation of hexagonal boron nitride (h-BN) nanoparticles and the formation of SiO₂/BN layer. Finally, complex oxidation layers with two distinct concentric sublayers accompanied with two transition sublayers could be formed after the oxidizing treatment.