Ultra-fine particles of protein lysozyme were prepared with a supercritical anti-solvent (SAS) apparatus by using dimethyl sulfoxide (DMSO) as solvent and carbon dioxide as anti-solvent. The influences of various experimental factors on the morphology and the mean size of particulate products were investigated. As evidenced from the experimental results, phase behavior of the mixtures in precipitator during the particle formation stage played a crucial role in the SAS processes. Uniform networked nano-particles were obtained when the,precipitations were conducted in the supercritical region of carbon dioxide + DMSO mixture. Several different types of morphologies were produced simultaneously as the precipitations were operated near the critical region. Spherical micron-scale clusters were formed in the superheated vapor region. while submicron-particles were aggregated as dense cake when lysozyme precipitated in the vapor-liquid coexistence region. The wide angle X-ray scattering (WAXS) patterns indicated that both the raw lysozyme and the processed particulate samples were amorphous. The differential scanning calorimeter (DSC) thermograms showed that the dehydration peak was disappeared after SAS treatment. Moreover. the networked primary particles could be disintegrated and dispersed well in water through ultrasonication, which were confirmed by analysis with dynamic laser scattering (DLS). A continuous stirred tank reactor (CSTR) model was used to calculate the dynamic composition variations of the mixtures in precipitator during the particle formation period. (C) 2008 Published by Elsevier B.V.