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, 1H NMR and 13C 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°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°C in argon, giving out amorphous silicon carbon nitride (SiCN) ceramics with a high ceramic yield of 76 wt%. 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°C and the formation of active N–Si bonds.