Flexible pavements are susceptible to the damaging effects of moisture, causing various kinds of problemes for asphalt such as stripping. That reduces the durability and serviceability life of pavements and consequently increases the construction and maintenance cost. The aim of this research is to study the moisture sensitivity of the hot asphalt mixture with high-density polyethylene as an asphalt binder modifier. Asphalt 60/70 was mixed with several concentrations of high-density polyethylene (HDPE) ranging from 2% to 8% by bitumen weight using a high shear mixer at a temperature of 180 0C and a speed of 4000 rpm for 60 minutes. Penetration depth, softening point, rotational viscosity (RV), and scanning electron microscopy (SEM) tests were performed on both the conventional and HDPE-modified binders. Asphalt mixtures were designed according to the Egyptian specifications using the Marshall method. Conventional and HDPE-modified asphalt mixtures' moisture susceptibilities were evaluated through indirect tensile strength (IDT) and loss of stability tests. Results of scanning electron microscopy (SEM) showed that HDPE was homogeneously dispersed through the binder with no polymer cluster formations. Testing results revealed that adding high-density polyethylene at a concentration of 4% gives superior performance in most tests. Adding HDPE significantly improved the properties of asphalt binder, increased the hardness of the asphalt mixture and reduced the effect of moisture damage.