For both CO2 enhanced oil recovery and CO2 sequestration in oil reservoirs, the diffusion of injected CO2 into oil-saturated porous media is of great significance ill project design, risk assessments, economic evaluation, and performance forecast. However, the measurement of CO2 diffusion coefficient in liquid-saturated porous media under reservoir conditions has not been well-established due to the complications caused by density-induced natural convection in the CO2-oil systems. This paper presented a new method for measuring the effective CO2 diffusion coefficient in oil-saturated porous media under reservoir conditions. A core plug with the two end faces sealed was designed as the physical model for radial diffusion process. The measurement was conducted in a high-pressure diffusion cell with an oil-saturated core sample placed ill the middle and the remaining void space of the cell filled by high-pressure CO2 sample. A small-pressure decay technique was employed to record the pressure change of the gas phase during the diffusion measurement. Because the oil phase contained in the porous medium swells as CO2 diffuses into it, the experiment essentially involves both diffusion and swelling-induced convection. To describe these processes involved in the measurement, a mathematical model along with numerical solutions was derived. The effective diffusion coefficient was determined by matching the experimental pressure decay Curve with the corresponding mathematical model. The proposed method was demonstrated for oil-saturated Berea core samples at pressures ranging from 2.2 to 6.5 MPa. The measured pressure decay Curves showed good agreement with the mathematical predictions using the best-fitted effective diffusion coefficients. The derived method can be readily implemented in laboratories that can handle high-pressure fluids; thus, this study provides a tool for studying CO2 diffusion in oil-saturated porous rocks.