The amorphous SiO2 filled PTFE composite material were prepared by using the chemical rotary steam. The thermal and dielectric properties of the composites were measured. Also, the microstructure evolution of the composites was characterized by the Scanning electron microscopy (SEM). The effects of SiO2 with different grain size (4 μm,9 μm, 13 μm and 20 μm) on the microstructure and dielectric properties for PTFE SiO2 composite dielectric material were investigated. It was found that the density, the thermal expansion coefficient and dielectric constant increased with the grain size SiO2 increase, while the dielectric loss was decreased with the grain size of SiO2 increase. When the grain size of SiO2 was 20 μm, PTFE could be well coated on the SiO2 layers, and the composites exhibited proper thermal expansion coefficient (17.58×10-6/℃), dielectric constant (2.82) and low dielectric loss (0.001 2).