Flow induced crystallization is an important process during polymer solidification because it strongly influences the material morphology and properties. The melt of isotactic polypropylene crystallizes slowly in isothermal conditions whereas shear conditions speed up the crystallization process. The main objective of the presented research is the investigation of the shear-induced crystallization of isotactic polypropylene through a comparative analysis between isothermal and non-isothermal conditions in different shear regimes. A numerical simulation of crystallization kinetics of semicrystalline thermoplastics with a multiscale model is proposed to evaluate the nucleation and growth of spherulites, after identifying the material parameters needed to connect crystallization kinetics to the molecular material properties. This model is validated by using data from differential scanning calorimetry (DSC) and a rotational rheometry (RHEO).