In past decades, many attempts have been made to use water-soluble polymers as a mobility control agent to improve sweep efficiency of enhanced oil recovery processes. However, sensitivity of the thickening behavior of these polymers to some harsh conditions, such as high salinity, has cast serious doubt on their applicability in reservoir conditions. By expansion of nanotechnology, scientists discovered that nanoparticles can be utilized as thickening and rheology control agents in many polymer solutions. In this study, hydrophilic fumed silica is added to hydrolyzed polyacrylamide and sulfonated polyacrylamide solutions. The effect of the addition of nano silica on the thickening and stability of the polymer solutions is investigated at different silica and electrolyte concentrations. Augmentation of nano silica has been found to have small but different effects on the thickening behavior of the two polymer solutions. It causes a viscosity increase in sulfonated polyacrylamide solutions while its effect on hydrolyzed polyacrylamide is decreasing at lower silica concentrations and increasing at higher silica concentrations. By increases in electrolyte concentration, the effect of nano silica on viscosity diminishes, and a severe coagulation problem is observed. These results question the applicability of nano silica particles as a thickening additive for solutions of these high molecular weight polymers, unless some treatments are done on the silica surface.