Carbon dioxide(CO2) flooding is an efficacious method of EOR and it is a very complicated process, involving phase behavior. To master the performance of CO2 flooding and provide accurate data for designing oil development, a comprehensive investigation of CO2 flooding phase behavior and mechanism based on laboratory study was conducted. In order to get representative fluid samples of a reservoir, it was necessary that the right operation of mixing the separator oil and gas samples to match the bubble point pressure be carried out. The result from PVT experiments shows that for a certain reservoir, through a slight manipulation of the measured PVT properties including bubble point pressure, volume factor, swelling factor, solubility of CO2 and viscosity, it is possible to obtain regression curves, which can be used to estimated the PVT behavior for any hydrocarbon fluid-CO2 mixture of this reservoir. The result of slim tubule tests indicates that it is more appropriate to determine the minimum miscibility pressure of the reservoir fluid-CO2 mixture by the position of inflexion on the curve of oil recovery with flooding pressure than by reaching a special oil recovery point.