Flaxseed is a good source of plant protein for both human diet and animal feed. Extrusion process effectively improves the digestion of flaxseed. In the present work, the effects of extrusion parameters such as screw speed, moisture content, temperature and feed rate on the IVPD of the extruded flaxseeds were investigated. A central composite design was used with four controlled variables: screw speed (70, 120, 170, 220 and 270 r/min); moisture content (4%, 10%, 16%, 22% and 28%); temperature (84, 108, 132, 156 and 180 degrees C); and feed rate (25.4, 47.4, 69.4, 91.4 and 113.4 kg/h). The experimental values of IVPD measured from 69.46% to 77.43%. A second-order model was obtained to predict the IVPD. It has been found that moisture content and feed rate affect the IVPD more significantly compared with the temperature and screw speed. In the present study, the maximum digestibility was obtained when the screw speed, moisture content, temperature and feed rate were 120 r/min, 10%, 120 degrees C and 91.4 kg/h, respectively. The application of this analysis method for optimizing the extrusion process of flaxseeds is discussed. (C) 2007 Elsevier Ltd. All rights reserved.