Monte Carlo simulation has been applied to calculate the pVT relationship of CO2 + butane (n-C4H10 and i-C4H10) systems at 310.93 K and up to 9.5 MPa. CO2 is treated as a single-site molecule and butanes are treated as four-site molecules. The Lennard-Jones (12-6) potential is used as the site-site potentials and the combining rules proposed by Jorgensen et al. [W.L. Jorgensen, J.D. Madura, C.J. Swenson, J. Am. Chem. Sec. 106 (1984) 6638.] are adopted for unlike site pairs. The calculated results of the pVT relationship show good agreement with the experimental data [T. Tsuji, S. Honda, T.Hiaki, M. Hongo, J. Supercrit. Fluids 13 (1998) 15.] by introducing an intersite interaction parameter between unlike molecules. Furthermore, the radial distribution functions and the number of CO, and butanes around butanes are calculated as a fundamental information on the microscopic structures. It is found that the radial distribution functions and the number of CO2 and n-C4H10 around n-C4H10 are different from those of CO2 and i-C4H10 around i-C4H10.