Styrene butadiene rubber (SBR) vulcanizates were prepared using liquid polyisoprene (PI) and hydrogenated polyisoprene (HPI) as the plasticizers, and their structure and mechanical properties were investigated. The effect of the phase-separated structures of the SBR and the liquid polymers on the mechanical properties of the SBR vulcanizates was investigated. The constant physical properties were observed for the SBR vulcanizates during the thermal aging process when SBR and the plasticizer were miscible with each other in the vulcanizates.