Applications of molecular simulation to supercritical fluids (SCF) and to vapor-liquid phase transition are described with emphasis on SCF carbon dioxide and Lennard-Jones (LJ) model fluids. From extensive Monte Carlo (MC) calculations for LJ fluid mixtures, it is found that the excess molar enthalpies of the model mixture simulating nonpolar mixture are positive and larger in SCF region as compared with those in high pressure and low pressure/room temperature regions. Their molar composition dependence cannot be expressed by the regular solution equation. A large-scale molecular dynamics (MD) simulation has been carried out with more than 55,000 LJ molecules in order to verify whether the so-called van der Waals loop which is seen in small-scale simulation will disappear. II is seen that use of larger system size and of low surface energy model lead to the disappearance of the loop. Finally, the application of quantum chemical molecular orbital (MO) calculation to the preparation of two-body potentials for carbon dioxide-solute is briefly discussed.