Composites of integrated 1-D MnO2 nanowires and 2-D graphene sheets at nanoscale are successfully prepared under the mild condition of 100 A degrees C. The fabricated materials are extensively characterized by electron microscopy and X-ray diffraction, and the formation mechanism is investigated. It is of particular note that the graphene sheets in this case play dual roles, both as active reagent to reduce MnO4- to form 1-D MnO2 nanowires and as active component of the composites integrated into the 3-D structure. The proof-of-concept demonstration shows that the 3-D composites can be used as active materials for supercapacitors, where the high-surface area 2-D graphene sheets serve as both high-surface area active materials and conductive supports for high-capacity 1-D MnO2 nanowires.