A numerical investigation on the extrusion cooking process for cereals in a co-rotating twin-screw extruder was carried out, using a fluid-dynamic, numerical simulation model. Simulation tests were carried out, varying temperature, screw rotation velocity, mass flow rate and extruder geometry. Fluid-dynamic parameters inside the extruder, as shear rate, residence time and mixing index were evaluated. Extruder geometry is an important parameter in the extrusion process, since it affects the values of shear rate and residence time and consequently the quality of the final product. Screw with regular geometry shows a more regular profile of the mean shear rate along the extruder, but characterized by lower values of this parameter, determining a reduced gelatinization of the compound. Instead, extruders characterized by a more complex geometry allow to obtain higher values of shear rate, but with some difficulties in the control of the final quality of the products due to a more complex behavior of the shear rate profile along the screw axis. (c) 2004 Elsevier Ltd. All rights reserved.