Zinc oxide nanoparticles (ZnO-NPs) and modified calcium carbonate (nano-CaCO3) nanoparticles were successfully prepared and added to polystyrene-butadiene rubber copolymer (PSBR) matrix to prepare PSBR nanocomposites. The prepared nanomaterials (ZnO-NPs & nano-CaCO3) were characterized using scanning electron microscope (SEM), transmission electron microscope (TEM) and X-ray diffraction (XRD). Furthermore, the prepared polymer nanocomposites and oil base mud were used for drilling in high pressure high temperature (HPHT) wells. The consequence of using polymer nanocomposites based on different loading of ZnO-NPs and nano-CaCO3 on the rheological properties of oil base mud was evaluated and enhanced the yield point at high pressure high temperature wells (HPHT). The using of the polymer with different percentage from (0.5) in all percent the obtained results is very promising; this means that the increase of polymer is reasonable for the increase of apparent viscosity, plastic viscosity and yield point at high temperature. Correspondingly, polymer nanocomposites displayed rise of apparent viscosity, plastic viscosity, and yield point, decreased in fluid loss and increased in electrical stability at high pressure high temperature wells.