Abstract: Perhydropolysilazane ( PHPS) and polyhydridomethylsilazane ( PHMS) were used as preceramic polymers , respectively , to prepare three-dimensional silica f iber reinforced silicon nit ride composites through repeatedinfilt ration and pyrolysis at a low temperature (600 ℃) , and the mechanical properties and microst ructures of thecomposites were investigated through comparison. The composite prepared with PHPS , with density of 1. 83 g/cm³ , porosity of 10 % , and flexural st rength of 45. 4 MPa , shows a smooth f racture surface and st rong f iber/ mat rixbonding , while the composite prepared with PHMS exhibit s a rough f racture surface and moderate interfacialadhesion , lower density of 1. 66 g/ cm³ , higher porosity of 16 % , but higher flexural st rength of 56. 3 MPa. Thedifferent activities of the precursors result in different interfacial microst ructures and mechanical properties of thecomposites.