Enhanced oil recovery plays an even important role in petroleum industry because there are fewer newly discovered oil reserves and, on the other hand, there is still approximately 50-60% of the original oil in place left in the existing reservoirs after primary recovery and second recovery. In enhanced oil recovery by water flooding, the composition of the connate and invading brines could have a major influence on wettability and consequently improve the oil recovery at reservoir temperature. In enhanced oil recovery by CO2-flooding, the presence of salts in water reduces the solubility of CO2 in water. As indicated in a simulation study, the oil recovery increases with the salinity of the brines. Despite of their important role, the modeling of brine/seawater under reservoir conditions is a challenging task. We proposed Statistical Associating Fluid Theory (SAFT) coupled with Restricted Primitive Model (RPM) to represent brine/seawater up to 473.15K and 1000 bar. Water and each ion (cation or anion) are modeled as a spherical molecule. The dispersive interactions between water and ion and between ions are taken into account. One set of parameters at 298.15K for 5 cations (Li+, Na+, K+, Ca2+, Mg2+) and 7 anions (Cl-, Br-, I-, NO3-, HCO3-, SO42-, CO32-), and temperature-dependent parameters for 5 cations (Li+, Na+, K+, Ca2+, Mg2+) and 5 anions (Cl-, Br-, HCO3-, SO42-, CO32-) are correlated from the experimental data of aqueous single-salt solutions in the temperature range of 298.15 to 473.15K and at saturation pressure of water. The properties of brine/seawater are then predicted up to high temperatures and pressures.