Borate-crosslinked guar is commonly used in hydraulic fracturing. The stimulation process conditions occur at temperatures between100-300 °F and high pressures up to 8000 psi. The viscosity of the borate-crosslinked guar shows interesting rheological responses under the influence of mechanical shear, pH and temperature. It has been found recently that the fluids significantly decrease in their viscosity under high pressure. This process was found to be reversible and very rapid. The lab measurements illustrate that the percentage loss in viscosity of guar/borate system could reach to 97 % at 8000 psi. Thus, the fluid may loss some or all viscosity contributed by the crosslinker. This thesis focusses on the addition of novel nanoparticle-based additives which could be added to a guar/borate crosslinked gel to minimize viscosity reduction under high pressures. The additives studied are zirconium oxide, titanium oxide and cerium oxide nanoparticles. This study shows that the percentage loss in viscosity could be reduced from 97 % to only 67 % when using these additives.