Phase behaviors of polydisperse polystyrene (PS)/ nematic liquid-crystal systems [Pethoxy -benzylidene -p -n-butylaniline (EBBA)] are investigated with a thermo-optical analysis technique. We also develop a thermodynamic framework to describe the phase behaviors of polydisperse PS/EBBA systems.Theproposedmodelisbased on a modified double-lattice model to describe isotropic mixing and MaierSaupe theory for anisotropic ordering. To correlate the polymer chain length and energy parameters in a nematic-isotropic biphasic region and to apply the primary interaction parameter in an isotropic-isotropic phase-transition behaviors of polydispersePS/EBBAsystems.Theproposed model shows remarkable agreement with experimental data for the model systems in comparison with an existing model.