The influence of in situ thermal reduction of graphene oxides (GO) on the isotactic polypropylene (iPP) crystallization were systematically investigated by comparing the crystallization behavior of iPP with different annealing procedures at 200 degrees C. Polarizing optical microscopy and differential scanning calorimetry results show that, for iPP/GO composite after annealing, both the number of nucleation sites and the rate of crystallization decrease with the increase of annealing time, while this effect cannot be observed for neat iPP. Comparative experiments on iPP containing reduced GO demonstrate that the crystallization behavior of iPP cannot be affected by annealing if GO is already highly reduced. Further investigation by temperature dependent Fourier transform infrared spectra indicates that the negative influence of annealing on iPP/GO crystallization is associated with the decrease in the degree of conformational ordering during the process of crystallization. Our results suggest that the melt annealing can alter the crystallization behavior of iPP with GO and this alteration is probably caused by the thermal reduction-induced chemical changes on GO sheets. (C) 2014 Elsevier Ltd. All rights reserved.