Dodecyl amine (DA) functionalized graphene oxide(DA-GO) and dodecyl amine functionalized reduced graphene oxide (DA-RGO) were produced by using amidation reaction and chemical reduction, then two kinds of well dispersed DA-GO/high-density polyethylene (HDPE) and DA-RGO/HDPE nanocomposites were prepared by solution mixing method and hot-pressing process. Thermogravimetric, X-ray photoelectron spectroscopy, Fourier transforms infrared spectroscopy, X-ray diffractions, and Raman spectroscopy analyses showed that DA was successfully grafted onto the graphene oxide surface by uncleophilic substitution and the amidation reaction, which increased the intragallery spacing of graphite oxide, resulting in the uniform dispersion of DA-GO and DA-RGO in the nonpolar xylene solvent. Morphological analysis of nanocomposites showed that both DA-GO and DA-RGO were homogeneously dispersed in HDPE matrix and formed strong interfacial interaction. Although the crystallinity, dynamic mechanical, gas barrier, and thermal stability properties of HDPE were significantly improved by addition of small amount of DA-GO or DA-RGO, the performance comparison of DA-GO/HDPE and DA-RGO/HDPE nanocomposites indicated that the reduction of DA-GO was not necessary because the interfacial adhesion and aspect ratio of graphene sheets had hardly changed after reduction, which resulting in almost the same properties between DA-GO/HDPE and DA-RGO/HDPE nanocomposites. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 39803.