Not only obtaining nano-sized particles, but controlling mono-dispersed nanoparticles has been regarded as one of the important techniques to employ nano-engineering in many disciplines. To fractionate the nanoparticles synthesized, the gas expanded liquid system (GXLs) has proven to be very useful and effective. Many researchers considered the total interaction energy model comprised as a summation of van der Waals attractive potential, the elastic repulsive potential, and the osmotic repulsive potential as a promising thermodynamic model. In previous models, osmotic contribution was modeled based on the rigid lattice model. Consequently, it was impossible to consider the effect of pressure on GXL operation because osmotic repulsive potential based on rigid lattice modal intrinsically could not reflect the pressure influence. We applied a lattice fluid model in the presence of holes to derive better osmotic repulsive potential. Thus, the effect of pressure on nanoparticle synthesis in GXL process has been successfully investigated. A nanoparticle size predicted using this improved model is in a better agreement to that obtained experimentally.