Oil well cementing is one of the most important steps in drilling and completion processes and providing a full zonal isolation which is the most important features in the oil cementing. Traditionally, Portland cement is used for oil cementing operations, however, a few years ago, a new cost-effective material came to light called geopolymer. In this research, a fly ash class C based geopolymer was used. This research investigates the effect of drilling fluid contamination with geopolymer cement to understand its impact on geopolymer rheological and mechanical performance. Initially, the optimized geopolymer was prepared and mixed with different drilling fluid ratios, including 0%, 5%, and 10% by weight of cement at atmospheric pressure and ambient temperature (24 °C). Same percentages were added to Portland cement to compare it with the geopolymer. Four tests were conducted to determine the effects of drilling fluids including: rheology, density, fluid-loss, and compressive strength for different curing time (1 day, 3 days, and 7 days). Results showed that drilling fluids enhanced the geopolymer rheological behavior by improving geopolymer viscosity and reducing the fluid loss. Drilling fluids reduced the geopolymer viscosity which facilitates the pumping operation during the cementing. In contrast, mixing the drilling fluid with Portland cement had a negative effect on rheological behavior as well as fluid loss. Mixing the drilling fluid with Portland cement increased the fluid loss significantly. In term of compressive strength, as the amount of drilling fluids increased, the compressive strength of geopolymer was not significantly affected. After 3 days curing time, geopolymer lost about 8.5% of its strength when 5% of drilling fluids weight percent was added. Whereas, Portland cement has lost 38.4% of its strength when 5% of drilling fluids weight percent was added. After 7 days curing time, geopolymer lost about 23% of its strength when 5% of drilling fluid by weight of cement was added. However, Portland cement lost 49% of compressive strength in the same conditions. These obtained results indicates that geopolymer has the ability to withstand the drilling fluids contamination.