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In this paper, a new interfacial thickness-based method, namely, the diminishing interface method (DIM), is developed to determine the minimum miscibility pressures (MMPs) of light oil–CO2 systems in bulk phase and nanopores. First, a Peng–Robinson equation of state (PR-EOS) is modified to calculate the vapor–liquid equilibrium in nanopores by considering the effects of capillary pressure and shifts of critical temperature and pressure. Second, the parachor model is coupled with the modified PR-EOS to predict the interfacial tensions (IFTs) in bulk phase and nanopores. Third, a formula of the interfacial thickness between two mutually soluble phases is derived, based on which the novel DIM is developed by considering two-way mass transfer across the interface. The MMP is determined by extrapolating the derivative of the interfacial thickness with respect to the pressure (∂δ/∂P) T to zero. It is found that the modified PR-EOS coupled with the parachor model is accurate for predicting the phase behavior and IFTs in bulk phase and nanopores. More specifically, in nanopores, the lighter components prefer to be in vapor phase by increasing the temperature or decreasing the pressure and the IFTs are decreased with the pore radius, especially at low pressures. The determined MMPs of 12.4, 15.0, and 22.1 MPa from the DIM agree well with the laboratory measured results for the three Pembina light oil–CO2 systems in bulk phase at T res = 53.0 °C. Moreover, the MMPs of the Pembina and Bakken live oil–pure CO2 systems in the nanopores of 100, 20, 4 nm are determined from the DIM, which tend to be decreased at a smaller pore level. Physically, the interface between the light oil and CO2 diminishes and the two-phase compositional change reaches its maximum at the determined MMP from the DIM.