A new mixing model for cubic equations of states that is theoretically correct in the composition dependence of the second and third virial coefficients is derived. This new mixing rule has been tested in conventional vapor-liquid equilibria calculations, It was found to be as good as activity coefficient models and other equation of state mixing rules for solutions of various kinds of nonideality. It can also be extrapolated over large temperature and pressure ranges. Its salient feature is an extra degree of freedom that accounts for ternary interaction, which enables it to give consistent correlation of vapor-liquid and liquid-liquid equilibria of ternary systems and the corresponding binaries.