Highly stable graphene aqueous dispersions were achieved by chemical reduction of graphene oxide with an environmentally friendly reagent of hexamethylenetetramine (HMTA). By this method, chemical reduction as well as dispersion of graphene can be carried out in one step without the need of organic stabilizers or pH control. The as-synthesized products were characterized by Fourier transform infrared spectroscopy, ultraviolet-visible spectroscopy, X-ray diffraction, Raman spectroscopy, atomic force microscopy, scanning and transmission electron microscopy, and thermogravimetry and differential scanning calorimetry. It is revealed that the bulk of the oxygen-containing functional groups were removed from graphene oxide via HMTA reduction, and stable aqueous colloidal dispersions of graphene have a concentration up to ca. 0.65 mg/mL. Moreover, it is found that the freshly precipitated graphene nanosheets can be re-dispersed in water with simple ultrasonic treatment. A mechanism for the formation of stable graphene colloidal dispersions is proposed. This simple and green approach should find practical applications in the preparation of graphene-based nanocomposites with a facile and low-cost solution processing technique. (C) 2010 Elsevier Inc. All rights reserved.