AbstractAs an organic solvent, naphthenic oil is used widely in the polymer industry, so it has the potential to be used with polymer to improve the performance of asphalt binder in pavement engineering. The effects of naphthenic oil as an additive for styrene-butadiene-styrene (SBS)–modified asphalt were investigated in this paper. Penetration, softening point, ductility, rheology property before and after aging, morphology observation, and storage stability were tested on naphthenic oil/SBS-modified asphalt. The following findings were achieved: Naphthenic oil increases the consistency and temperature sensitivity of SBS-modified asphalt. It is remarkably beneficial to the low-temperature property of SBS-modified asphalt. It is not advantageous to the resistance to deformation. Naphthenic oil is beneficial to the antiaging property of SBS-modified asphalt and helps the polymer to get swollen, proving that naphthenic oil has some properties of aromatics. Another discovery is that naphthenic oil makes the SBS distribute more evenly, which is consistent with the discovery in the rubber industry. Future investigation of its use with other additives and the anti-ultraviolet light (UV) aging properties are suggested.