Supercritical fluid extraction of chamomile using carbon dioxide was investigated in the current study. A model that accounts for both particle and fluid phase was presented for the supercritical extraction. The distribution coefficient of chamomile extract, between solid and solvent has been determined using genetic algorithm method. The model was solved numerically and was successfully validated with experimental data. The model was found to give superior results when compared to experimental data. The effect of particle diameter on extraction yield was investigated using the proposed model. Using genetic algorithm optimization technique 313.15 K and 20 MPa were found as the optimum temperature and pressure, respectively. (C) 2010 Elsevier B.V. All rights reserved.