During tertiary phase of oil production, the interactions between reservoir rock and fluids (i.e., oil and brine) are intentionally modified to favor oil flow through porous rocks and recover more oil from reservoirs. Wettability of rock surface plays an important role in deciding oil recovery from oilfields. This work presents an experimental investigation of wettability alteration of natural sandstone and carbonate rock surfaces by silica nanoparticles in high salinity conditions. Commercial silica nanoparticles (SNP) were first chemically modified for its stability in high salinity conditions and subsequently were employed for wettability alteration studies. Wettability was evaluated by measuring contact angles between natural and model surfaces and oil in brine environment. The results showed that neutral-wet surfaces were turned water-wet and substantial change in contact angles of oil with rock surfaces were observed.