The effect of temperature, pressure and flow on relaxation time (or spectrum), crystallization time, nucleation density and rate, crystallites growth rate, the interrelation among these quantities and the distributions of deformation rate and cooling time during the process determine the morphology distribution in the final object. Rheology in its turn undergoes tremendous changes during crystallization. A simple model linking all these quantities was developed to describe morphology evolution during polymer processing. The effect of flow on nucleation density and growth rate of an iPP is described on the basis of a molecular stretch parameter; the stretch evolution is described by a simple nonlinear Maxwell model whose relaxation time, in its turn, is determined by the molecular stretch and, obviously, temperature pressure and crystallinity. The model is applied to the description of morphology evolution during injection moulding of a very accurately characterized iPP as far as rheology, quiescent crystallization and effect of flow on nucleation and spherulitic growth rates. The comparison with the data of pressure evolution during the process reproduces the effect of injection flow rate, holding pressure and mold temperature. Also main characteristics of final morphology are reproduced by the simulations.