The carbon dioxide (CO2) injection method has been used to enhance oil recovery from Chinese reservoirs. Reservoir fluid was first prepared by combining the gas and liquid phase using the gas-oil ratio data. The influence of CO2 molar composition (x(CO2)) on the interfacial tension of the CO2 injected crude oil + reservoir water under stratum conditions was studied using a pendant drop method, where x(CO2) was from (0, 10.0, 34.1, 44.7, 48.9, 57.8, to 65.0) mol %. The bubble point pressure for these CO2 injected oil systems was also determined using a RUSKA PVT device, which could be used to keep the measured system under single-phase conditions. The experimental data showed that when x(CO2) changed from (0 to 65.0) mol %, the interfacial tension value decreased by about one-third. The pressure had a slight effect on the interfacial tension. When x(CO2) was 65.0 mol %, the C-O2 injected oil system approached complete miscibility and the interfacial tension data of C-O2 injected crude oil + reservoir water changed a little with an increase of pressure.