A "green'' route to ultrahigh-temperature Si-B-C-N ceramic from vacuum-degassing waste gas of polyborosilazane {B[C2H4Si(CH3) NH](3)}(n) (T2-1) has been developed. After gas-to-gel transformation, an amorphous ceramic Si5.3B1.0C19N3.7 was derived from the gel by dehydrocoupling and polymer-to-ceramic transformation. The ceramic started to form a nano-structure at 1700 degrees C and resisted thermal degradation up to 2200 degrees C in argon. This suggests that vacuum-degassing waste gases of polymer precursors may be perfect raw materials for various advanced ceramics.