Shear thickening fluids (STFs) are non-Newtonian fluids whose viscosity increases abruptly above a critical shear rate. At high shear rates, microstructural transformations occur that lead to a liquid-solid transition as a result of hydrodynamic interactions. This behavior makes STFs suitable for many industrial, biomedical, and military applications. Silica is one of the materials that has been used widely for preparing STFs. Silica-based STFs are commonly prepared by dispersing silica particles into polyethylene glycol (PEG). However, the thickening behavior of the STFs significantly depends on many factors, including the physical and chemical properties of the dispersed particles, the shear history of suspension, temperature, and the presence of additives. In this study, we investigated the effect of size, surface chemistry and concentration of silica nanoparticles (NPs) on the rheological behavior of STFs. The suspensions were prepared at different NP concentrations of 5 to 30 wt. %. The results showed that thickening behavior is very sensitive to the investigated parameters. The critical shear rate and extent of shear thickening increased by increasing the NP loading. Besides, non-flocculated suspensions of silica/PEG showed the greatest extent of thickening. As the final step in this investigation, synthesized graphene oxide was added to the silica/PEG suspensions to analyze the effect of 2D additives on the rheological behavior of STFs. It was observed that addition of a small amount of graphene oxide, as low as 0.1 wt. %, improves the thickening behavior. These results provide us with unique information to design and fabricate STFs with desired final properties for advanced applications.