Liquid polycarbosilanes substituted by unsaturated groups were prepared by a one-pot synthesis with Cl2Si(CH3)CH2Cl, Cl3SiCH2Cl, and CH2=CHCH2Cl (or HC CMgBr) as the starting materials. The as-received polycarbosilanes were characterized by gel permeation chromatography, Fourier transform infrared spectroscopy, and nuclear magnetic resonance and were confirmed to have hyperbranched Structures. The polymer yield, Si-H content, side reaction, unsaturated group content, and polymer structure are affected by the amount of the starting materials in feed. The thermal properties of the polycarbosilanes were investigated by thermogarvimetric analysis. The ceramic yield of allyhydridopolycarbosilane is about 60% and that of ethynylhydridopolycarbosilane is over 75%, indicating that the liquid polycarbosilanes have great potential to be promising precursors to SiC. Moreover, the compositions of the polycarbosilanes and the final ceramics can be tailored by controlling the amount Of the starting materials. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113:1611-1618, 2009