density-dependent one-fluid mixing rule for the size parameter is proposed from Carnahan-Starling equation and Boublik-Mansoori equation using a numerical method. This mixing rule coincides with the mixing rules usually used at zero pressure and at infinite pressure. It also tends to Carnahan-Starling equations for pure fluids at limiting conditions, such as infinite dilute solution, infinite diameter ratio and equal diameters. Its application to equations of state is also carried out for Lennard-Jones mixtures including binary and ternary mixtures. Compared with the van der Waals one-fluid mixing rule, distinct improvement is obtained, especially for predictions of PVT, chemical potential and vapor-liquid equilibrium data.