Straight asphalt binders have been modified by addition of both high-density polyethylene (HDPE) and a blend of HDPE and ethylene-propylene-diene-monomer (EPDM). The blend composition was fixed to 90/10 HDPE/EPDM to illustrate the possibility of adapting the polymer to be added to the asphalt binder for specific end-use applications. Linear viscoelastic properties of unmodified and polymer modified asphalts at concentrations ranging from 1 to 5 wt% were studied before and after Thin-Film Oven Test (TFOT) aging. Temperatures ranging from −15°C to 60°C were considered. Standard tests such as Ring-and-Ball softening point, Fraass breaking point and TFOT aging were also performed on the whole set of samples. It was found that addition of rubber-modified polyethylene (HDPE/EPDM) to the straight asphalt results in materials with enhanced overall properties, and most important, dispersed phase much more stable than the equivalent HDPE modified asphalt, mainly before TFOT aging. Good results were obtained for 1% HDPE/EPDM samples. Optimum design is, however, required for the desired properties to be obtained.