A simple instability condition is applied to delineate the regions of unstable metal flow in the processing map utilising flow stress data for AFNOR 7020 aluminium alloy, which was fitted in a constitutive equation. Unstable regions identified from the simple instability condition at different strain levels were superimposed in the processing map. This phenomenon takes into account the dependence of strain rate sensitivity and strain hardening effect on plastic instability during hot deformation of the material. The processing map has been developed and validated by comparing microstructural observations of deformed compression specimens.