To study the thermal stability of polymer derived SiBNC ceramics, a polyborosilazane was pyrolyzed in N 2 and NH 3 /N 2 atmosphere, respectively. The as-pyrolyzed products were annealed in N 2 atmosphere at 1200–1850°C for 2h. The chemical composition and phase structure of the as-pyrolyzed and annealed ceramics were investigated by element analysis, XRD and FT-IR. The results show that all the ceramics exhibit excellent high temperature stability. They are fully amorphous to 1700°C, and only partial crystallization, giving a mixture of Si 3 N 4 , BN and SiC phases, is observed upon heating at 1850°C. The N 2 pyrolyzed products show better stability than the NH 3 /N 2 pyrolyzed products and have less tendency to crystalline at higher temperatures. Better retention of nitrogen at high temperatures is also observed for the N 2 pyrolyzed products.