The manufacturing of the high expansion linear polypropylene (PP) foams has long been considered a challenging task due to the low melt strength and the weak rheological behavior of the linear PP. In this study, a reinforced PP with polybutylene terephthalate (PBT) nanofibrillar composite was used to improve the foamability of PP; we also showed how the viscosity of PBT affects the nano-fibril morphology. Nano-fibrillation is a novel technology that was recently introduced in foaming. Other works have shown that by introducing PET nano-fibril into the PP matrix, the expansion ratio and the cell density of PP can be significantly improved. However, due to the high viscosity of PET and the low affinity between PP and PET, those PET fibrils have large diameters. Thus, an alternative method is needed to further improve the conventional nano-fibrillar composite for the foaming purposes. In this study, three kinds of PBTs with different viscosity were used to reinforce PP via melt spinning. The results show that nano-fibril (~150 nm) were formed in all the samples. The composites with low PBT concentration have a “bead-like” structure along the nano-fibril. This phenomenon is believed to be caused by the low matrix viscosity and the nonuniform stretching of the polymer melt. The oscillatory shear behavior was studied by comparing the storage modulus (G’) and the phase angle (tanδ) of the non-fibrillated and the fibrillated samples. Extensional viscosity was measured to illustrate its enhancement on the melt strength. Differential scanning calorimetry was applied to study the crystallization kinetics of PP/PBT fibrillar composites. Finally, a customized batch foaming equipment was used to make the microcellular foams.