The effects of terminal groups and composition of hyperbranched polymers (HBPs) on the crystallization behavior of poly(ethylene terephthalate) (PET) in HBP/PET blends were investigated using in situ FT-IR spectroscopy and differential scanning calorimetry. Hyperbranched aliphatic polyester was synthesized from an AB(2) type monomer and a B-3 type core. The hydroxyl terminal groups of the HBP were transformed into acetate and benzoate groups by organic synthetic methods. The effects of the HBPs were compared with those of linear polyester (LPE) synthesized from an AB type monomer and a Bz type core. A small amount of HBP with hydroxyl groups could greatly reduce the relative crystallinity of PET through hydrogen bonds. HBP with acetate groups showed effects similar to those of LPE. As the composition of LPE was increased, the relative crystallinity of PET was slightly decreased. On the contrary, HBP with benzoate groups played the role of nucleating agent in the blends with PET. The crystallization rate of PET was highest in the blend with 12 wt % HBP. Dynamic viscosity measurements were performed to investigate the role of the HBPs as processing aids.