CO2 flooding can substantially improve recovery while achieving carbon storage，maintaining a balance between economic benefits and environmental protection. Accordingly，it is necessary to research the diffusion law of CO2 in reservoirs. Experiments of CO2 diffusion in oil-saturated cores were conducted with self-designed HTHP diffusion sample holders. A mathematical model was established for calculating the diffusion coefficient of CO2 in oil-saturated cores. The measured pressure curves were fitted with the theoretical pressure curves to determine the CO2 diffusion coefficient. In addition，the influence of permeability on CO2 diffusion in oil-saturated cores was analyzed. The research results demonstrate that the mathematical model can faithfully reflect the CO2 diffusion law in oil-saturated cores，and the measured diffusion coefficients are highly accurate. The diffusion coefficient of CO2 in oil-saturated cores is at the order of magnitude of 10-8 m2/s within the experimental range. The diffusion coefficient of CO2 in cores increases with the rise in permeability，but the increment declines.