We investigated the effects of silica nanoparticle (NP) on nanofluid flooding for enhanced oil recovery. All NPs used in experiments were identically surface modified with PSS-co-MA and a zwitterionic surfactant. In core flooding experiments, the oil production from Berea Sandstone showed an increasing trend as the NP concentration increased within the range of 0.1 to 2.0wt%. This result was closely associated with variance of interfacial tension (IFT) and contact angle (CA). IFT continued to decrease as the NP concentration increased until 2.0 wt%. However, IFT cannot further decrease with an increase in the NP concentration beyond 2.0 wt%, because total interaction energy may be reduced due to the decrease of electrostatic repulsion force by the closer spacing between NPs. When combined with silica nanofluid soaking, the CA of the rock/oil/nanofluids increased with increasing NP concentration; this indicated wettability alteration to a more water-wet condition caused by an enhanced fluid ability to spread silica NPs along the rock surfaces. Because of this effect, the capillary pressure is expected to be sufficiently reduced by nanofluid flooding, compared with brine flooding. However, at higher NP concentration, the NPs caused permeability reduction and an increased pressure drop attributable to the residual NPs in rock pores. This result implies additional oil recovery attributable to improved sweep efficiency related to the log jamming phenomenon caused by the residual NPs, as well as the IFT reduction and wettability alteration, thus leading to enhanced oil recovery.