The determination of minimum miscibility pressure (MMP) of CO2/oil in tight oil reservoirs has aroused huge attention from researchers. In this study, we characterized the mass transfer process and determined the MMP of CO2/oil systems in nanoslits through molecular simulation. A bulk phase and five confined nanoslits with the pore size ranging from 2 to 15 nm were examined. It was found that the MMPs in nanoslits were significantly lower than those in bulk phase, thus revealing that rising the probability of molecular interaction is a vital factor affecting the miscible phase. Furthermore, due to the consideration of adsorption layer, the width of nanoslit with the lowest MMP was larger than the reported value, which will bring more uncertainty to the molecular migration. This simulation study highlighted the crucial role of the confinement effect in the miscible process of CO2 flooding, which might facilitate the development of low permeability reservoirs. (C) 2019 Elsevier Ltd. All rights reserved.