This project covers a complete in-depth study of crystallization behavior of PP and in-situ fibrillated PET composites. Two kinds of PET, APET and CPET, were examined. SEM and DSC experiments were carried out to exam fiber morphology and crystallization kinetics. PET nanofiber was manufactured through a scalable melt spinning technology, and around 250 nm PET fiber was obtained. Isothermal and non-isothermal DSC experiments were performed. With the presence of PET nanofiber, dramatic improvement of PP’s crystallization kinetics was observed. Both Avrami and Mo’s method indicated the crystallization complexity and crystallization rate were increased. Reprocessing of fibrillated material into pellet showed moderate setback effect in PET’s promotional effect in PP crystallization. Also, the mechanical property was measured to show the influence of changed crystal structure. Increase in both the modulus and strength of the composites was observed.