Particle packing, together with other filler characteristics, can be considered as an important structural parameter of fillers, affecting on the properties of rubber compounds. For this purpose, N220 and N550 carbon blacks with nearly similar structure and surface chemistry but different particle sizes were selected. N220/N550 showed the maximum packed density at the composition 35/65 according to the accepted theory for bimodal packing of spheres. A correlation between the packing behavior of carbon blacks mixtures and physical, electrical and rheological behaviors of filled rubber compounds was investigated. A synergic effect in compounds properties was observed before the vulcanization reaction at the maximum packed composition of carbon-blacks. The synergic effect was attributed to the dense and highly immobilized rubber trapped inside packed structure of carbon blacks aggregates, but it didn't appear for the vulcanizates due to the adverse effect of immobilized rubber on the chemical cross-link density formed during vulcanization reaction. • Particle packing is an important structural parameter in rubber reinforcement. • There is a correlation between particle packing and the properties of filled rubber compounds. • A synergic effect in properties was observed at the maximum packed composition of fillers before vulcanization. • The synergic effect was due to the highly immobilized rubber trapped in the filler network. • The synergic effect was absent in some properties after vulcanization.