Due to lack of previous study on applying polymer additives in road base construction, this research presents experimental results on the improvement of long-term road base performance by the addition of carboxylated styrene–butadiene emulsion (Rovene® 4045) and Portland cement. The specimens stabilized with Portland cement (0–6%) and Rovene® 4045 (5–10%) and then subjected to different stress sequences to study the wetting and drying (WD) and wheel tracking (WT) tests on the 7-day-cured specimens. Results of tests conducted to assess the specimens’ resistance to WD cycling showed that the addition of a 4% Portland cement–7% Rovene® 4045 mixture resulted in reductions of 86.99% in water absorption, volume changes of 88.55%, and weight changes of 92.84% relative to a sample with only 4% cement after 12 WD cycles. The permanent strain behaviour of the samples was assessed by the WT test. The results of WT test showed that the permanent deformation characteristics of the mixture were considerably improved by utilization of Rovene® 4045 modification, which resulted in reductions of 218.9% at 25 oC, and 356.8% at 50 oC in permanent strain of the mixture respectively. Therefore, this research nominates a new polymer additive having outstanding engineering properties and environmental friendly.