In this study, rapid expansion of supercritical solution (RESS) was used to micronize the chitosan particles. Design of experiment was implemented with response surface methodology (RSM) using Mini Tab software 17. The operating temperature (30-70 degrees C), pressure (7-22 MPa) and effective nozzle diameter (500-1200 mu m) were considered as the range of operating variables while spraying distance (1 cm) and nozzle length (5 mm) were held constant. The characterization (size and morphology) of the precipitated particles of chitosan was determined by scanning electron microscopy (SEM). The results show great reduction in the size of the precipitated particles of chitosan compared with the original particles. A decrease in the particle size of chitosan was found with increasing extraction pressure and temperature along with a decreasing of nozzle diameter. Response surface analysis verified that R-2 and modified R-2 of the model for 500 mu m effective nozzle diameter were 97.19% and 95.18%, respectively. (C) 2016 Elsevier B.V. All rights reserved.