In the last decades one of the main efforts was to tailor by the process the final properties of the molded object. It is quite well known that morphology distribution in the molded object influences the final properties of the part. In particular to obtain reliable predictions of the final properties, a correct description of the different morphologies development (namely fibrillar and spherulitical) during the injection molding process is required. To this aim evolution of fibrillar structures during step shear rate experiments carried out in a Linkam shearing cell was observed. Experiments were performed at different temperatures using several shear rates on a well characterized isotactic polypropylene. An overall model for rheology, crystallinity and molecular stretch evolutions during the shearing tests was developed. In particular crystallinity evolution was described by coupling two competitive crystallization processes: one for the spherulitical morphology and one for the fibrillar structures. For the spherulitical structure a model including Flow Induced Crystallization (FIC) effects was adopted whereas for fibrillar structure a new model was developed. Predictions obtained with the proposed overall model were able to satisfactorily describe the development of fibers during the shearing experiments.