Mn3O4/graphene nanocomposites were synthesized by mixing graphene suspension in ethylene glycol with MnO2 organosol, followed by subsequent ultrasonication processing and heat treatment. The as-prepared product consists of nanosized Mn3O4 particles homogeneously distributed on graphene nanosheets, which has been confirmed by field emission scanning electron microscopy and transmission electron microscopy analysis. Atomic force microscope analysis further identified the distribution of dense Mn3O4 nanoparticles on graphene nanosheets. When used as electrode materials in supercapacitors, Mn3O4/graphene nanocomposites exhibited a high specific capacitance of 175Fg−1 in 1M Na2SO4 electrolyte and 256Fg−1 in 6M KOH electrolyte, respectively. The enhanced supercapacitance of Mn3O4/graphene nanocomposites could be ascribed to both electrochemical contributions of Mn3O4 nanoparticles, functional groups attached to graphene nanosheets, and significantly increased specific surface area.