Abstract: Using fabric of silica fiber and silica sol as the raw materials, the silica fiber reinforced silica based composite materials (SiO_2f/SiO₂) were fabricated by the sol-gel method, and the silica fiber reinforced silica and boron nitride based composite materials (SiO_2f/SiO₂-BN) were prepared via the urea route. The mechanical properties and dielectric properties of the composites were investigated. The results show that the density of the composites increases with the increase of the cycles, while the densification becomes slow gradually. After three cycles, the density of the SiO_2f/SiO₂-BN composites reaches 1.81 g/cm³. The flexural strength and elastic modulus are 113.9 MPa and 36.5 GPa, which increase by 12.1% and 76.3% compared with SiO_2f/SiO₂ composites, respectively. The interfacial combination between fiber and matrix is good and lots of fiber pull-out can be observed on the fracture surface, which suggests a good toughness. The composite has excellent dielectric properties with the dielectric constant and loss tangent being 3.32 and 0.004, respectively.