In this work, crystallization behavior of polypropylene (PP) containing in-situ fibrillated polyethylene terephthalate (PET) nanofibril was examined. Material was prepared in three steps, blending, fibrillation, and reprocessing. Plain PP, PP fiber containing fibrillated PET, and reprocessed PP pellets containing fibrillated PET were examined and the results were compared. Polarized microscopy results revealed the promotional effect of PET nanofibril in polypropylene crystallization. Rod like crystal structure was observed with fiber presence, indicating active crystal nucleation and growth along the fibers. Isothermal differential scanning calorimetry (DSC) qualitatively and quantitatively proved that higher crystallinity and better crystallization kinetic were achieved by adding PET fibril. Crystallization onset time was push earlier. The crystallization half time was also significantly decreased. Further, the rheometer test showed increased melt viscosity at low frequency range, which indicated that polymer chain movement was impinged by the percolated fibre network and crystals.