Abstract: Si-B-N fibers were prepared by melt-spinning, curing and pyrolysis under high temperature of a precursor polyborosilazane(PBSZ) under NH₃ atmosphere, then the fibers were heated up to ultra-high temperature under purity N₂. The ultra-high temperature resistance property of the fibers was researched by elemental analysis, FTIR, XRD, SEM, mechanical properties analysis and wave-guide methods. The results show that the weight of Si-B-N fibers lose little annealed at 1 500 ℃, the fibers can remain amorphous up to 1 700 ℃ in inert atmospheres, and a mixture of Si₃N₄ and BN crystalline phases are observed upon heating to 1 850 ℃; average tensile strength and average elastic modulus of Si-B-N fibers are 1.72 GPa and 196 GPa at room temperature respectively, and they are 1.86 GPa and 205 GPa annealed at 1 500 ℃, respectively. So the fibers exhibit excellent ultra-high temperature resistance property. In addition, the Si-B-N fibers show good dielectric properties according to the measurement by a wave-guide method, average dielectric constant and dielectric loss tangent of the Si-B-N fibers annealed at 1 400 ℃ are about 3.68 and 0.001 1 in 8-12 GHz, respectively.