During the twin extrusion process, starch basic structure is modified and it is very important to be able to predict the changes experienced by the product. To model the transformation of starch during twin screw extrusion, it is thus necessary to take into account the strong coupling between viscosity and thermomechanical treatment. It is the purpose of the present work. We have used the software Ludovic (c) to calculate the flow of a molten starch along a twin screw extruder. We have assumed that the degradation reaction (viscosity decrease) was linked to the specific mechanical energy received during the flow. We have compared extrusion cases with and without viscosity/energy coupling, and we show that it is important to take it into account to correctly predict the parameters of the extrusion process (torque, energy, product temperature, etc.) and the level of starch transformation. POLYM. ENG. SCI., 50:1758-1766, 2010. (C) 2010 Society of Plastics Engineers