The effects of polymorphic form and particle size of SiO2 fillers on the dielectric, mechanical and thermal properties of SiO2–Poly-etheretherketone (SiO2–PEEK) composites were investigated in this paper. Strong low frequency (<10Hz) Debye-like dielectric dispersions could be observed for all samples. The dielectric permittivity at high frequencies of the composite exhibits little morphology or particle size-sensitive characteristics of the SiO2 fillers. All the composites obtained in this case demonstrate the dielectric permittivities of ~3.5 at high frequencies. The crystalline a-cristobalite filled composite exhibits lower dielectric loss and mechanical strength, but larger thermal expansion coefficient and thermal conductivity, compared with the similar particle sized amorphous SiO2 filled one. The crystalline a-quartz filled composite demonstrates the lowest mechanical strength and high-est dielectric loss. An increase in particle size of the spherical fused silica fillers decreases the dielectric loss, while increases the thermal conductivity of the composite. The flexural strength of the composite reaches the maximum value of 113.MPa when the particle size of spherical SiO2 filler is ~10 μm. Particle packing by combining optimal amounts of differently sized spherical fused silica fillers leads to a substantial improvement of mechanical strength (153MPa) coupled with reasonable dielectric and thermal properties due to the synergic effect (dielectric permittivity (εr) = 3.35, dielectric loss (tanδ) = 1.63 × 10.3 @10 GHz, thermal conductivity (l) = 0.74 W/m*k (90 °C), coefficient of thermal expansion (a) = 23.6ppm/°C and relative density (r) = 99.72%).