A novel approach to fabricate covalently functionalized graphene oxide (fGO)/polypropylene (PP) nanocomposites has been reported. Graphene oxide is modified with p-phenylenediamine and cyanuric chloride and then grafted with maleic anhydride grafted polypropylene (MAPP). The fGO achieves good dispersion with exfoliated and intercalated nanostructure and strong interfacial adhesion in PP. A significant enhancement of thermal stability of the nanocomposites is obtained at low fGO loading, such as a 133 degrees C increase in initial decomposition temperature and a 94 degrees C increase in the temperature at maximum rate of weight loss in the nanocomposite with 1 wt% of fGO. The mechanisms for improving the thermal stability are fully demonstrated. The high content of fGO (1 wt%) can act as a beta-nucleating agent for PP crystallization. The addition of only 0.5 wt% fGO increases the storage modulus and heat deflection temperature (HDT) of PP by 15.4% and 11 degrees C, respectively. However, the elongation at break of nanocomposites are decreased and the value of tensile strength show no change with increasing loading of fGO, which is in contrary to those of the nanocomposites as expected. The orientation parameters of samples before and after stretching are studied to illustrate the mechanism. The reduction of mobility of lamellae by fGO is the main reason for the decrease in ductility. The possible mechanism for tensile strength is presented. The orientation of polymer chains and lamellae with respect to the direction of action of the force during deformation are inhibited in the presence of fGO. (C) 2013 Published by Elsevier B.V.