Owing to their high-temperature stability and microwave-transparent properties, SiBN fibers are promising reinforcement materials for microwave windows or radomes in harsh environments. This work investigates the high-temperature properties of continuous SiBN fibers with different boron contents. Compared with Si3N4 fibers, the SiBN fibers demonstrated superior high-temperature stability, retaining their original tensile strength up to 1600 degrees C with a smooth surface and amorphous structure. The boron-containing microstructure improved the microwave-transparent properties of the SiBN fibers by decreasing their dielectric constant and loss. The values were stabilized at temperatures up to 1200 degrees C. The high-temperature stability and microwave transparency of the SiBN fibers were likely contributed by the synergistic effect of the amorphous BN phase and the Si-N-B networks (the main existing state of the boron atoms).