Nanoparticles of the metallocene catalyzed cyclic olefin copolymer (mCOC) were prepared with a lab scale continuous supercritical anti-solvent (SAS) apparatus. Several experimental parameters were investigated to explore their influences on the morphology and the mean size of the products. These parameters included the character of carbon dioxide injector, precipitation pressure and temperature, anti-solvent density, the polymer concentration, the flow rate of feed, and the nature of organic solvents. The phase states of the mixtures of solvent + anti-solvent in precipitation stage may govern the mean size of the resulting particles. Nanoparticles of mCOC were obtained in the homogeneous compressed liquid or supercritical region, while sub-micrometric particles were formed in the vapor-liquid coexistence region. The morphology of precipitates was also significantly changed, turning from spherical nanoparticles to microfibrils, as the polymer concentration of feed was increased from 2 to 4 wt.%. Under 15 MPa and 306.2 K, the mean size of the resultant primary mCOC particles as small as 39 nm with a standard deviation of about 6 nm could be produced. The residual solvent was about 1.6 wt.% in the sample after SAS processing by analyzing with a thermogravimetric analyzer (TGA), and the particle samples exhibited sub-glass transition behavior according to the thermal histograms of differential scanning calorimeter (DSC). (c) 2006 Elsevier B.V. All rights reserved.