Polymer-derived SiBCN, with superior thermal stability and amorphous activity, was introduced into ZrB2 powders. This sintering aid highly improved the sintering efficiency of ZrB2 ceramics at medium temperature (1000-1600 degrees C), which showed a different service temperature range from that of traditional crystal additives. The microstructure and densification behavior of ZrB2-SiBCN samples were mainly studied. The polymer structural evolution including construction, rearrangement, and crystallization of the amorphous SiBCN network, made a large contribution to the densification of ZrB2 ceramics. The carbothermal reduction of pyrolysis carbon with oxide impurities could not only decrease the oxygen content, but also develop the activity of chemical bonds in SiBCN network. Diffusions and reactions at the interface also benefited the microstructure and consolidation of ZrB2-SiBCN ceramics.