Monte Carlo simulation has been used to obtain excess potential energies, Helmholtz energies of mixing, local concentration ratios and cluster size distribution of Stockmayer/Lennard-Jones (ST/LJ) fluid mixtures as a simple model of polar-nonpolar fluid mixtures at three state points including the supercritical state. The results of the calculations for the Helmholtz energy of mixing show that the ST(mu*(2) = 4)/LJ mixture will become immiscible in the region about x(ST) = 0.333 at T* = 1.15, rho* = 0.822. Also, the asymmetric behaviors of U-E* with respect to the ST composition are explained in terms of the ’difficulties in breaking ST clusters’ and ’orientational frustration relaxation’.