. As many oil fields are going into their final stage of production, the new technologies are necessary in order to maintain the production and increase the recovery of hydrocarbons. The chemical injection methods as Enhanced Oil Recovery techniques (EOR) are used to change the rock and fluid condition to increase oil recovery. In recent years, the Nano fluid injection has been introduced as an efficient and invironmentally friendly approach. The purpose of this research is to investigate the Nano particle effect on carbonate rock wettability alteration and determination of optimum nano particle concentration. Four different concentration of nano silica dispersed in 30 times diluted sea water were used. The spontaneous imbibition tests and contact angle measurements were conducted at 75 . The Nano fluid with 0.05% concentration has shown 11% increase in oil recovery compared to same diluted sea water without nano silica. According to the results, the presence of the connate water in the rock negatively affect oil recovery during Nano fluid injection. Moreover nano silicadid not change the wettability of sandstone rock. Keywords: Silica nano fluid, diluted sea water, wettability, carbonate rock, enhanced oil recovery