According to effects of greenhouse gases especially Carbon Dioxide at atmosfer such as increase the average temperature of earth over recent years, CO 2 storage is done in subsurface reservoirs for long-term. In this study, have been investigated CO 2 sequestration process in subsurface reservoirs. CO 2 can be stored at Geological formations containing oil and gas reservoirs or deep saline aquifers. In the first part of the study, have been investigated CO 2 Sequestration process in aquifer. the CO 2 sequestration Process in aquifer, a numerical isothermal, two dimensional and two-phase model was considered. a wetting phase (brine-rich phase) and a nonwetting phase (CO 2 -rich phase) exists. They each contain the components water and CO 2 that The solubility of water in the nonwetting phase is neglected. After writing mass balance equations for each phase, equations are discretization using finite difference method and simultaneous were solved with numerical methods. Finally, after simulation a sample reservoir using MATLAB and CMG software, variations of pressure, solubility CO 2 in wetting phase and wetting phase of saturation were predicted and compare with each other. In the second part, has been investigated the process of CO 2 injection into oil reservoirs. a numerical isothermal, three-phase and two-dimensional model was considered that containing oil phase, gas phase and water phase. After writing mass and momentum balance equations, a compositional model is applied to achieve The final reservoir pressure and saturation equations that They were discrete using finite difference method. For solving these equations, is applied explicit saturation-implicit pressure formulation. The model results were compared with experimental data. The results showed that the final recovery in samples laboratory 54% is volume of voids, but in this study is 62%. Finally, after checking certainly of the models accuracy, analysis was process of performance at one of Iranian oil reservoirs.