Thermal and morphology properties of polyamide-12 (PA12)/poly(butylene terephthalate) (PBT) blends with hyperbranched poly(ethyleneimine)-g-polyamide-12 (PEI-g-PA12) as reactive compatibilizer were studied by a combination of Optical Microscopy, DSC and C-13 NMR. This compatibilizer was synthesized by simple amidation reactions. The addition of PEI-g-PA12 greatly modified the morphology of PA12/PBT blends, which were originally an incompatible polymer pair. At the beginning of the addition of compatibilizers, the sizes of PA12 and PBT rich phases decreased and the blends dispersed better. Then the phase structures became more diffused with time. At last, bi-phase morphology disappeared totally and one homogeneous structure was obtained at 260 degrees C. The higher the PEI-g-PA12 compatibilizer content was, the faster the morphology changed. Our DSC measurements provided a consistent picture of the crystallization behavior of PA12 and PBT components. The comparison of 13C NMR spectrums between PA12/PBT and PA12/PBT/PEI-g-PA12 undergoing the same thermal treatment indicated that compatibilization was originated from the formation of hyperbranched PBT-co-PEI-co-PA12 copolymers. (C) 2009 Elsevier Ltd. All rights reserved.