In this paper, a comprehensive research including laboratory experiments and theoretical models is conducted to investigate the diffusion behavior of supercritical CO2 in micro- and nanoconfined pores. First, a total of five diffusion tests with five different permeability core samples are conducted to determine CO2 diffusion coefficients in porous media. Second, a diffusion model, which comprises a straightforward physical model and a series of mathematical formulations, is developed to evaluate the diffusion process in micro- to nanoconfined pores coupled with an improved equation of state. Core samples and reservoir fluids are specifically characterized. The micro- and nanometer-scale pores are found to be complex in the pore structure with high textural coefficient and tortuosity. The phase behavior of reservoir fluids is found to substantially change when the permeability and pore radius are less than 0.001 mD and 0.1 mu m, respectively. The CO2 diffusion in the crude oil-saturated micro- and nanoconfined pores is categorized as the bulk and Knudsen diffusion, whose diffusion coefficient is determined from the pressure-decay method. More specifically, the CO2 diffusion coefficient is increased with the increase in permeability and pore radius. Furthermore, the reduced permeability/pore radius lower than 0.1 mu m leads to a smaller diffusion coefficient by including the critical shifts at the same pore scale.