Tremendous amounts of hydrocarbons are located in deeper formations. In deep formations we experience higher pressures and temperatures. Designing a proper drilling fluid that can tolerate such high-pressure, high-temperature (HP/HT) conditions is very challenging. This work is focused on investigating the rheological behavior of water-based drilling fluids with different properties at extremely high pressure and temperature conditions using a state-of-the-art viscometer capable of measuring drilling fluids properties up to 600°F and 40,000 psig. The results of this study show that the viscosity, yield point and gel strength decrease exponentially with increasing temperature until the mud samples fail. This behavior is the result of the thermal degradation of the solid, polymers, and other components of the mud samples. Increase in the distance between molecules due to high temperature will lower the resistance of the fluid to flow and, hence, its viscosity, yield point, and gel strength will reduce. Moreover, conducting this study led to the conclusion that viscosity, yield point, and gel strength increase linearly as the pressure increase. Pressure's effect on these parameters, however, is more apparent at lower temperatures. Ultimately, the study concluded that the mud samples that were used, which are standard industrial types, failed at a temperature of 250°F and that the combined effect of temperature and pressure on mud's rheology is complex.