In this work, Zr-modified polyborosilazanes (PZC-PBSZ) with a high zirconium content over 10wt% were synthesized via radical reaction between Cp₂Zr(CH₂CH=CH₂)₂ and liquid polyborosilazane under the existence of dicumyl peroxide. The structure and reaction mechanism of PZC-PBSZ were studied using Fourier transforminfrared spectra, nuclear magnetic resonance and X-ray photoelectron spectroscopy. The results shown that besides the self-polymerization, Cp₂Zr(CH₂CH=CH₂)₂ can also reacted with Si-H in LPBSZ via hydrosilylation reaction and substitution reaction to form Zr-C and Zr-Si, respectively. The thermogravimetric-mass spectrometry and ₂₉Si MAS NMR results shown that the escape of goups such as Cp, (CH₃)₃Si and (CH₃)₂SiH was ooccured at temperature range from 400 _oC to 800 _oC. As the temperature went up to 800~1200_oC, the polymer PZC-PBSZ converted into inorganic mineral, and the evolution of SiHN₂C and SiN₃C structure to SiN₄ structure was occurred at that temperature. After pyrolyzing at above 1000 _oC, PZC-PBSZ can convert into ZrC/SiBNC ceramic nanocomposites, with ZrC nanoparticles finely dispersed in SiBNC matrix. The obtained ZrC/SiBNC ceramic nanocomposites can remain fine and nanocrystalline microstructure after annealing at 1800 _oC.