Graphene oxide (GO), NaF and CaCl2 were mixed in their aqueous solutions to obtain a GO-CaF2 hybrid with CaF2 particles uniformly dispersed on GO surface, and further in situ reduced by hydrazine to obtain reduced-GO (RGO)-CaF2 hybrid. RGO-CaF2/brominated butyl rubber (BIIR) composites were obtained by solution mixing, in which RGO-CaF2 was well dispersed in nano-sized as confirmed by transmission electron microscope images. The RGO-CaF2/BIIR composites had much better mechanical properties than BIIR. The stress at 300% extension of BIIR increased 72% after the addition of 3 parts per hundreds of rubber (phr) RGO-CaF2. The improved mechanical properties benefited from the strong interaction between RGO and BIIR. The dielectric constant of RGO-CaF2/BIIR composites increased with increasing RGO-CaF2 loading. Because of the synergistic effect of RGO and CaF2, the stresses at various extensions, dielectric constant and thermal conductivity of RGO-CaF2/BIIR composites were higher than those of RGO/BIIR or CaF2/BIIR composites. The improved mechanical, dielectric and thermal properties bring BIIR the potential to be used in a wider range of application areas.