Polyborosilazane, a low viscosity preceramic polymer with good infiltration efficiency and high char yield, was used to prepare amorphous composites with polymer derived nitride matrix reinforced by 2?5-dimentional silica fibre, and the mechanical properties and microstructures were investigated. The composites without fibre coating showed typical brittle fracture behaviour with a smooth fracture surface and a longitude flexural strength of just 46?3 MPa whereas the composites with fibre coating exhibited non-brittle fracture behaviour with distinct fibre pull-out on the fracture surface and a high longitude flexural strength of 129?5 MPa. It was the controlled fibre/matrix interface by precoating treatment that contributed to the high mechanical property of the composites.