Nano-particles of pigment red 177 were prepared with a continuous supercritical anti-solvent (SAS) apparatus by using dimethyl sulfoxide (DMSO) as a solvent and supercritical carbon dioxide as an anti-solvent. The effects of several key factors on the particle size were investigated. These factors included the inside diameter of capillary tube, concentration of the pigment solution, the flow rate of the solution, and temperature, pressure and density in the precipitator. The experimental results showed that smaller inside diameters of capillary tube, lower concentrations of pigment solution, lower densities but higher than the critical values of mixtures of carbon dioxide + DMSO are favorable to produce smaller particles for pigment red 177. With the aid of experimental design methods, the mean particle size of the primary pigment powders can be processed to as small as 46 nm. (C) 2004 Elsevier B.V. All rights reserved.