Surface modification of silica particles, synthesized using the Stöber method, was carried out using methyl trimethoxy silane (MTMS) and 3-aminopropyltriethoxy silane (APTES) to prepare silica particles with hydrophobic and hydrophilic surfaces, respectively. Shear thickening fluids (STFs) were prepared from unmodified and surface-modified silica particles with polyethylene glycol as the carrier fluid. Rheological analyses of these STFs revealed that STFs prepared from APTES modified silica particles showed much higher peak viscosity as compared to STFs prepared from unmodified silica particles (control). However, the STF prepared with MTMS modified silica particles exhibited much lower peak viscosity than the control STF. Para-aramid (Kevlar) fabrics treated with the MTMS modified silica based STF showed better yarn pull-out force and impact energy absorption as compared to fabrics treated with APTES modified silica based STF and to the control STF. The reason for such behavior can be attributed to changes in the distribution patterns of STFs in Kevlar fabrics due to changes in the interactions between the hydrophilic Kevlar fabric substrate and the surface modified silica particles, which either become hydrophobic or hydrophilic based on the type of silane used. The results show that the surface modification of silica particles can be used as an approach to tune the impact performance of STF treated aramid fabrics.