The significance of severe plastic deformation by equal channel angular pressing (ECAP) as a promising technique for extreme grain refinement down to sub-micrometer and sometimes nanometer scale is generally recognised. We present a modelling frame for describing ECAP based on microstructure evolution. Following a particular scenario of grain refinement, in which a dislocation cell structure is considered as a 'precursor' of the developing grain structure, the variation of the cell size with the number of ECAP passes is traced. Finite element simulations based on the model compare favourably with the experimental data. Further features of the model such as a provision for modelling the variation of the misorientation angle distribution and texture evolution are also discussed.