Carbon dioxide (CO2) flooding is one of the very important industrial processes for enhanced hydrocarbon recovery. Therefore, a better understanding of the CO2 solubility and its effect on hydrocarbon swelling is vital for a successful CO2 flooding project. In this study, CO2 solubility in octane and its effect on octane (n-octane) swelling are investigated by performing configurational-bias Monte Carlo simulations in the osmotic ensemble at two temperatures of 323 and 353 K and a pressure range of 2-10 MPa. This study provides a quantitative understanding of the relationship between the CO2 solubility and the octane density reduction/swelling. The simulated swelling factor of the CO2 saturated octane is linearly correlated to the CO2 solubility. The results are in good agreement with the linear correlation derived from the published experimental data. Our results have also indicated that the interaction between the octane and CO2 is the main cause of the octane swelling. Molecular modeling proves to be a valuable tool for the calculation of gas solubility in the hydrocarbon and its effect on the swelling factor, and this method is applicable to a wide range of gas-oil systems.