Si-based ceramics derived from polymer precursors allows ceramic structure designing from molecular level to avoid the high temperature sintering process as well as the control of microstructure by sintering aids.The amorphous structure resultant from the pyrolysis at around 1000℃ could sustain a large degree of thermal stability above 1400℃ in Si-C-N and even above 2000℃ in Si-B-C-N,albeit in nitrogen.High-resolution TEM study could not give a clue to understand the origin of such an amorphous structure.