A study of the microstructural evolution of Zircaloy-4 (Zy-4) during hot extrusion has been performed on a small scale device mounted on a standard MTS machine. Before extrusion the billets present a Wind-manstätten lamellar microstructure. Additional data have been obtained by hot torsion tests. After hot deformation some samples have been heat treated in various conditions. Thus the fragmentated volume fraction has been quantified with optical microscopy for various strain, strain rate and temperature histories, focusing on post dynamic effects (activation and kinetics of grain growth after deformation). For describing the microstructural evolution, the model separates dynamic and post dynamic effects. The dynamic part uses a Gourdet-Montheillet approach to describe the fragmentation. The growth is described by an explicit differential Avrami approach which can model the non isothermal evolution during cooling after extrusion. The first calculations with this model provide a good prediction of the recrystallized volume fraction for several thermal and mechanical conditions. © 2008 Verlag Stahleisen GmbH, Düsseldorf.