Particle design using supercritical CO2 has been of great interest in the pharmaceutical, microelectronic, catalytic, and related industries over the past 10 years. The solubility of most drug compounds in carbon dioxide is very low, making it a very attractive antisolvent for particle formation. Particles from Solution enhanced dispersion by supercritical fluids (SEDS) is one of the supercritical antisolvent fluid techniques for nanoparticles generation, This process uses a coaxial nozzle design where the solvent with the solute of interest is injected in the inner tube and the supercritical CO2 is injected in the outer tube. The two streams mix in a small volume region of the nozzle called ‘mixing length’ before exiting through the nozzle tip into a chamber maintained The fast mixing process rapidly expands the solvent with CO2 in order to induce phase split of the solid drug particles. Main goals of this thesis were to model this process with using ANSYS FLUENT 13 software and model validation via comparison with experimental data. Furthermore, the validation model was used to investigate the effect of different design and operating variables on the particle size of the synthesis sized nanoparticles. Moreover, the parametric analysis of nanoparticle size design was carried out at the different operating range of effective variables. In this thesis by comparing results of model with a particular experimental data (pharmacicual nanoparticles of acetaminophen), the model predicted data had 1-30% error in contrast to experimental results. The other reported models in the literature have higher error than the model of this study. Thus, this model was used to investigate the effect of different parameters such as pre-expansion temperature, pre-expansion pressure and nozzle length on the average particle diameter and so we can use this model to know effect of dependent parameters to get profiles of thermodynamic profiles of thermodynamic properties along the nozzle and capillary. Dependent on entrance conditions to nozzle, average of particle diameter can be in the range of 0.7- 20 µm. The obtained narrow particle size distribution is a normal Gaussian curve which coincides with experimental data. Modeling results of Particles from Solution enhanced dispersion by supercritical fluids showed that; increasing the pre-expansion temperature produced largerer particles because temperature is an effective parameter on the growth of particles. The increasing pre-expansion pressure led to the synthesis of larger particles. Modeling results of SEDS showed that increasing nozzle length led to smaller particles, so that Lang mixing length of nozzle is appropriate (suitable) design condition. Keywords: modeling, Supercritical antisolvent fluid, acetaminophen, Solution enhanced dispersion by supercritical fluids, coaxial nozzle