A preceramic Polymer for Si-B-N-C fiber, polyborosilazane, has been synthesized by one-step condensation reaction of dichloromethylsilane, BCl3, and hexamethyldisilazane with high yield. The reaction mainly involves the condensation of Si-Cl and B-Cl with N-SiMe3 followed by SiMe3Cl evaporation and dehydrogenation between N-H and Si-H. The resulted polymer is a soluble colorless transparent solid with melting point of 70 C and molecular weight of 10,800. The backbone of the polymer is mainly composed of -Si-N-B-bridge with son e borazine rings. The polymer exhibits good processability and flexible Polymer fibers with diameter of 15-20 pm were obtained by melt spinning. Pyrolysis of the as-synthesized polymer to 1000 degrees C under nitrogen atmosphere results in a ceramic yield of 63 wt%, and the Obtained Si-B-N-C ceramic remains fully amorphous up to 1700 C, and only small amount of poorly crystallized BN, Si3N4, a Phases were observed upon heating at 1850 C. (c) 2008 Wiley Periodicals, Inc.