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Random inorganic networks composed of silicon, boron, nitrogen and carbon represent amorphous SiBNC ceramics with outstanding durability up to 1800℃[1].Various single source preceramic polymers polyborosilazane(PBSZ) have been synthesized and show crucial determination to the composition and properties of SiBNC ceramics[2].This work was aimed to study the Si/B atoms ratio of the PBSZ and the high temperature properties of the pyrolyzed products.For their moderate soften point, the spinnability also was tested by melt-spinning process.Three kinds of PBSZ polymers were obtained via ammonolysis reaction of the trichloroborane(TCB) and dichloromethylsilane(DCMS) with hexamethyldisilazane(HMDZ) and the mole ratio of TCB: DCMS: HMDZ is 1∶0.5∶6, 1∶1∶6 and 1∶2∶6, respectively.The prepared polymers and their derived ceramics were investigated with respect to their element contents and chemical structures, by means of elemental analysis and spectroscopy techniques including FTIR, MAS NMR and XPS.The backbone of PBSZ was mainly composed of-Si-N-B-with some borazine rings and as the mole ratio of DCMS went up, the Si/B ratio of PBSZ-1∶0.5∶6, PBSZ-1∶ 1∶6 and PBSZ-1∶2∶6 increased, correspondingly.Moreover, the soften points and TGA of the polymers with different Si/B ratios were studied.The results emphasized that with the increasing of the Si content in PBSZ, the soften points increased from 106.3℃(PBSZ-1∶0.5∶6) to 127.3℃(PBSZ-1∶1∶6) to more than 300℃(PBSZ-1∶2∶6) and ceramic yields increased as well from 41.4% to 50.8% to 62.3% correspondingly.Under optimized process, the continuous green fiber of about 20μm and loner than 1000m was obtained by spinning PBSZ-1∶1∶6 at 220℃.Above all, the results indicated that the PBSZ with equal Si and B would be the best precursor for SiBNC ceramic fibers.The fabrication of high performance SiBNC fiber is progressing steadily and will reported in detail later.