The composition，structure and properties of three kinds of ceramics obtained by the pyro1ysis of po1ysilazane (PSZ) and po1ycarbosi1ane (PC) have been studied. The pyro1ysis of PC to 1250℃ yie1ds Si-C ceramic，which is β-SiC microcrysta1，and the pyro1ysis of PSZ in N₂ or NH₃ atmosphere yie1ds Si-N-C and Si-N amorphous ceramics respective1y. The three kinds of ceramics were heat-treated between 1300℃ and 1500℃ in Ar gas to examine their therma1 stabi1ity，the result shows that Si-C ceramic changes from microcrysta1 to β-SiC crystalline and Si-N ceramic from amorphous to a-Si₃N₄，accompained by the observab1e weight loss whi1e Si-N-C ceramic retains its e1ementa1 composition and β-SiC microcrystalline states up to 1500℃.