Liquid density of long-chain iz-alkane systems from C-9 up to C-20 have been calculated using an analytical equation of state based on the statistical-mechanical perturbation theory. The equation of state consists of three temperature-dependent parameters: the second virial coefficient, a constant for scaling the softness of repulsive forces, and an effective hard-sphere diameter equivalent to the van der Waals covolume. The second virial coefficient of these n-alkanes are scarce and there is no accurate potential energy function to allow their theoretical calculation. In this work the second virial coefficients have been calculated using a corresponding-states correlation based on the surface tension and the liquid density at the freezing point. The other two temperature-dependent parameters of the equation of state can be calculated using a scaling rule. The calculated densities of these alkanes are within 5% of experiment.