A process model for the calculation of the microstructure evolution of nickel-based alloys during a hot forming process helps to optimise the forming process and to achieve the desired microstructure. In the grain structure model, the mean dislocation density rate within a homogeneous area can be estimated by the summation of a work hardening and a recovery rate. If a critical dislocation density is reached, recrystallisation nuclei will form. This critical dislocation density and the correspondent critical nucleus size can be calculated with a nucleation criterion for dynamic recrystallisation. During dynamic recrystallisation the nucleation process takes place at the grain boundaries of the deformed grains. The number of grains as a function of time is described by both a phenomenological and a physical based approach. The model was tested by a comprehensive experimental programme with the Alloy 80A.