An ethylene-bridged copolycarbosilazane precursor of copolysilylethylenediamine (co-PSDA) is synthesized by polycondensation of ethylenediamine with the mixture of vinylmethyldichlorosilane and methyldichlorosilane in the presence of triethylamine as acid absorbing agent. Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR, H-1 NMR and C-13 NMR) spectral analysis of the as-synthesized co-PSDA suggests a structure of ethylene-bridged polycarbosilazane having -Si-N-C-C-N- as backbone chain with -CH=CH2, -H and -CH3 attached to Si as side groups. Co-PSDA can be cross-linked at 80 degrees C using 2, 2-azobisisobutyronitrile as initiator through the polyaddition of the vinyl group and dehydrogenation/deamination of Si-H and N-H. Then the cross-linked co-PSDA precursor is pyrolyzed at 1000 degrees C in argon, giving out amorphous silicon carbon nitride (SiCN) ceramics with a high ceramic yield of 76wt%. The obtained SiCN ceramics consist of nitrogen-rich silicon sites of SiN4 as predominant component and some SiCN3 sites, which should arise from the breaking of N-C bonds below 600 degrees C and the formation of active N-Si bonds.