Rubber oil and thermoplastic elastomer styrene–butadiene–styrene (SBS) were selected for this work to study the distribution of rubber oil in SBS. Glass transition temperature (Tg) of different components in rubber oil and rigid or soft phase in oil-extended thermoplastic elastomer SBS were measured by means of rheometer and dynamic mechanical analysis. Results calculated by Fox equation and verification tests proved that rubber oil is not only distributed in soft phase but also in rigid phase of SBS. Saturated hydrocarbon components of rubber oil tend to dissolve in soft phase, while aromatic components tend to dissolve in rigid phase.