The microscopic mechanism of the volume swelling of CO2-alkane (decane, octane, hexane and cyclohexane) systems and the effects of temperature, pressure and alkane structure on the volume swelling of CO2-alkane systems are investigated by performing molecular dynamics simulation. It is shown that the increase in pressure, the reduction in temperature and the straight-chain structure of the alkane are of benefit to the volume swelling of CO2-alkane systems by calculating the volume swelling coefficient; CO2 in supercritical state plays a dominant role in the volume swelling of CO2-alkane systems. The microscopic process of the volume swelling of CO2-decane system shows that the increase in the average separation distance between decane molecules and the stretch of decane molecules result in the volume swelling of decane as CO2 dissolve into decane. The calculations of interaction energies in CO2-decane system indicate that the interaction between CO2 and decane molecules is responsible for the volume swelling of CO2-alkane system. Further study on the interaction between CO2 and decane molecules shows that the dispersion interaction, resulting in the different solubility of CO2 in alkanes, is the essence of the volume swelling for CO2-alkane system. This work is a good start on understanding the mechanism of alkane swelling influenced by CO2 at the molecular level and provides useful information for guiding CO2 enhancing oil recovery. (C) 2014 Elsevier Ltd. All rights reserved.