Abstract: It was pursued to compare simulation results in which CO₂ dissolution in the aqueous phase was either ignored or considered. The solubility law of CO₂ in the aqueous phase was investigated by using a CO₂-hydrocarbon-water equilibrium thermodynamic model. This compositional model combined with the Henry Law was then applied to a one-dimensional long-core simulator, in which a series of one-dimensional continuous CO₂ flooding was simulated with or without CO₂ dissolution in the aqueous phase at different initial water saturation. The difference caused by CO₂ flooding was discussed in terms of variation laws for oil recovery, gas to oil ratio, oil-gas-water saturated profiles and CO₂ mole fraction profiles in oils and formation water. The results showed that the amount of CO₂ dissolving in formation water increased with the increase of pressure but decreased with increasing temperature. The effect of pressure or temperature on the CO₂ dissolved amount in the aqueous phase became diminishing either as temperature rose to 100℃ or as pressure reached up to 20 MPa. During the early stage of gas injection, the oil recovery with the presence of CO2 dissolution was lower than that without the presence of CO₂ dissolution, moreover, with the presence of CO₂ dissolution the gas breakthrough time was later and oil banks toward to production wells were lagging behind, therefore, the higher the water saturation, the greater the effect of CO₂ dissolution. When water saturation was 0.67 and the CO₂ amount double of the hydrocarbon pore volume was injected, the ultimate oil recovery with the presence of CO₂ dissolution was 6% less than that without the presence of CO₂ dissolution. The effectiveness of CO₂ flooding might be delayed due to the CO₂ loss caused by CO₂ dissolution in formation water.