Carbon dioxide (CO2) injection is a successful enhanced oil recovery (EOR) technology that is being widely applied in North American oil fields. Studies have suggested CO2-based EOR is technically possible in the Middle Bakken Formation of the Williston Basin in North Dakota. The swelling of a crude oil/CO2 mixture plays a crucial role in the CO2 flooding process. Therefore, a better understanding of the effect of CO2 on crude oil swelling and viscosity reduction is critical for a successful CO2 EOR project in unconventional reservoirs such as the Bakken Formation. In this work, a series of n-alkane/CO2 systems were studied by performing configurational-bias Monte Carlo (CBMC) simulations and molecular dynamics (MD) simulations. The effects of carbon chain length, pressure, and temperature on the CO2 solubility and the swelling factor were investigated. The solubility of CO2 and the swelling factor of the CO2-saturated n-alkane are positively correlated to the pressure, while negatively correlated to the carbon chain length and temperature. With more CO2 dissolved, the interaction energy between n-alkane molecules becomes less negative, which indicates the swelling of the n-alkane/CO2 system. n-Alkanes with a longer carbon chain have a more negative intermolecular interaction energy and thus have a smaller swelling factor after saturation with CO2. With the increase of the CO2 mole fraction, the viscosity of the n-alkane/CO2 system is reduced. n-Alkanes with longer carbon chains have a larger viscosity reduction with increasing amounts of dissolved CO2. This study provides a reliable assessment of the volume swelling and viscosity reduction of n-alkane/CO2 systems and can further assist the CO2 EOR application in the Bakken Formation.