Gamma irradiated recycled high density polyethylene (gamma-HDPER) was investigated as a modifier in bitumen by means of the physical and chemical tests. Recycled high density polyethylene (HDPER) was exposed to gamma irradiation to form free radicals on the waste polymer that could provide a chemical bonding. The forming of new free radicals on polymers was observed by using Fourier transform infrared spectroscopy (FT-IR). The textures of the binders were obtained by means of florescent microscopy. Subsequently, in this study, physical test methods, roll thin film oven (RTFO), pressure aging vessel (PAV), rotational viscosity (RV), dynamic shear rheometer (DSR) and bending beam rheometer (BBR) tests were conducted on base and modified bitumens. The study shows that gamma-HDPER additive improves high temperature performance of bitumen and binder with gamma-HDPER can be used a larger temperature range than base bitumen.